[feat](trx-rs): redesign SDR noise blanker with tuning profiles

The old IQ noise blanker tracked a fast running RMS and, on a
threshold crossing, replaced the sample with the last clean one. That
hard sample-and-hold is a step discontinuity: it splatters energy back
across the wideband passband, so after the narrow channel filter it
often sounded worse than the noise it removed — especially on SSB, CW
and digital. It also had no look-ahead (the impulse leading edge leaked
through before the fast RMS reacted), blanked only single samples, and
used a fixed 1/128 time constant that did not scale with capture rate.

Redesign the blanker around accepted wideband-NB practice and add
profiles matched to the interference source:

- Noise-floor tracker updated only from clean samples and frozen while
  blanking, so a burst cannot desensitise detection.
- Detection on instantaneous power vs threshold² × noise floor.
- Look-ahead delay line so the gate closes *before* the impulse reaches
  the output, removing the leading edge.
- Raised-cosine tapered gate (ramp 1→0→1) instead of a hard hold, which
  minimises blanker splatter.
- Windowed blanking that re-arms on every detected sample to cover the
  full width of a burst.
- All timings expressed in real time and converted to samples at the
  capture rate, so behaviour is consistent across SDR sample rates.

Profiles (`NoiseBlankerProfile`, default `spike`): spike, ignition,
powerline, broadband — each selects the blank window, look-ahead, taper
and floor time constant. `threshold` stays orthogonal as the
sensitivity knob.

Core/protocol:
- New `NoiseBlankerProfile` in trx-core (serde/parse/u8/TS), re-exported
  at the crate root; `RigFilterState.sdr_nb_profile` for state sync.
- `profile` added to `RigCommand`/`ClientCommand::SetSdrNoiseBlanker`,
  the trait method, and the command mapping.

Config: `[rig.sdr.noise_blanker] profile = "spike"` (regenerated
trx-rs.toml.example).

SDR backend: `NoiseBlanker` rewritten in the channel DSP; profile wired
through `SoapySdrConfig`, the runtime setter, and `filter_state()`.

Frontend: an "NB profile" selector in the SDR advanced controls
(POST /set_sdr_noise_blanker&profile=…), reflecting server state; the
profile rides along with the enable/threshold quick toggle so it is
preserved. Regenerated generated.ts and app.js.

Tests: profile u8/parse round-trips (trx-core); DSP tests for impulse
suppression with no leading-edge leak, strong-steady-signal
pass-through, and wider-profile-blanks-longer.

Docs: User-Manual NB section rewritten for the new algorithm and
profiles.

Signed-off-by: Stan Grams <sjg@haxx.space>

docs/User-Manual.md

@@ -752,9 +752,30 @@ A dedicated tab with a clock icon provides:
 ## SDR Noise Blanker
 
 The noise blanker suppresses impulse noise (clicks, pops, ignition interference)
-on raw IQ samples before any mixing or filtering takes place. It works by
-tracking a running RMS level of the signal and replacing any sample whose
-magnitude exceeds **threshold x RMS** with the last known clean sample.
+on raw IQ samples before any mixing or filtering takes place — the only point in
+the chain where an impulse is still short in time, since the narrow channel
+filter downstream smears it into un-removable ringing.
+
+It combines four elements:
+
+- A **noise-floor tracker** — an exponential estimate of the background level,
+  updated only from clean samples (and frozen during a blank) so a burst cannot
+  drag the reference up and blind the detector.
+- **Detection** — a sample is flagged when its power exceeds
+  **threshold² × noise-floor**.
+- **Look-ahead** — the stream is delayed a few microseconds so the gate can
+  begin closing *before* the impulse reaches the output, catching its leading
+  edge instead of letting it leak through.
+- A **tapered gate** — instead of a hard sample-and-hold (which splatters energy
+  back across the passband and is what made the old blanker sound worse on
+  SSB/CW/data), the gain ramps smoothly down and back up, so blanking costs only
+  a short, quiet notch.
+
+The blank window, look-ahead, taper, and floor time constant are chosen by a
+**profile** matched to the interference source. The `threshold` control is
+orthogonal — it sets detection sensitivity within the chosen profile. All
+profile timings are specified in real time and converted to samples at the
+capture rate, so the blanker behaves consistently across SDR sample rates.
 
 ### Configuration (server-side)
 
@@ -771,6 +792,7 @@ type = "sdr"
 [rigs.sdr.noise_blanker]
 enabled = true
 threshold = 10.0     # 1 – 100; lower = more aggressive blanking
+profile = "spike"    # spike | ignition | powerline | broadband
 ```
 
 For the legacy single-rig (flat) config the path is `[sdr.noise_blanker]`:
@@ -779,15 +801,30 @@ For the legacy single-rig (flat) config the path is `[sdr.noise_blanker]`:
 [sdr.noise_blanker]
 enabled = true
 threshold = 10.0
+profile = "spike"
 ```
 
-| Field       | Type  | Default | Range   | Description |
-|-------------|-------|---------|---------|-------------|
-| `enabled`   | bool  | false   | —       | Turn the noise blanker on or off. |
-| `threshold` | float | 10.0    | 1 – 100 | Multiplier applied to the running RMS. A sample whose magnitude exceeds this multiple is replaced. Lower values blank more aggressively; higher values only catch strong impulses. |
+| Field       | Type   | Default   | Range   | Description |
+|-------------|--------|-----------|---------|-------------|
+| `enabled`   | bool   | false     | —       | Turn the noise blanker on or off. |
+| `threshold` | float  | 10.0      | 1 – 100 | Multiplier applied to the tracked noise floor. A sample whose magnitude exceeds this multiple is blanked. Lower values blank more aggressively; higher values only catch strong impulses. |
+| `profile`   | string | `"spike"` | see below | Tuning profile matched to the interference source. |
 
 The noise blanker is off by default.
 
+### Profiles
+
+Each profile sets the blank-window width, look-ahead, gate taper, and
+noise-floor time constant. Pick the one that matches what you are hearing, then
+fine-tune with the threshold.
+
+| Profile      | Blank window | Best for |
+|--------------|--------------|----------|
+| `spike`      | Narrowest    | Sharp, sparse impulses — ignition sparks, static crashes, keyed relays. The safe default: minimal impact on the wanted signal, good for SSB/CW/digital. |
+| `ignition`   | Medium       | Automotive ignition, electric fences, PWM/LED drivers — clusters of medium-width pulses at a high repetition rate. |
+| `powerline`  | Wide         | Power-line and arcing noise — buzzy bursts locked to the 100/120 Hz mains cycle. Uses a slower floor tracker to ride out the burst. |
+| `broadband`  | Widest       | Dense, continuous impulse noise where suppression matters more than fidelity. Most aggressive gating; expect some softening of the wanted signal. |
+
 ### Choosing a threshold
 
 The threshold controls how aggressively the blanker suppresses impulses.
@@ -813,39 +850,44 @@ the running average signal level.
 
 ### Web UI
 
-When the server reports noise-blanker support, two controls appear in the
+When the server reports noise-blanker support, these controls appear in the
 **SDR Settings** row of the web interface:
 
 - **Noise Blanker** checkbox — enables or disables the blanker in real time.
+  The **N** keyboard shortcut toggles it too.
 - **NB Threshold** number input (1–100) with a **Set** button — adjusts the
-  detection threshold. Press Enter or click Set to apply.
+  detection sensitivity. Press Enter or click Set to apply.
+- **NB profile** selector — chooses the profile (Spike / Ignition / Powerline /
+  Broadband). Changing it applies immediately.
 
-Both controls stay hidden until the server sends filter state containing NB
+The controls stay hidden until the server sends filter state containing NB
 fields, so they only appear when connected to an SDR backend.
 
 ### HTTP API
 
 ```
-POST /set_sdr_noise_blanker?enabled=true&threshold=10
+POST /set_sdr_noise_blanker?enabled=true&threshold=10&profile=spike
 ```
 
 | Parameter   | Type   | Required | Description |
 |-------------|--------|----------|-------------|
 | `enabled`   | bool   | yes      | `true` or `false` |
 | `threshold` | float  | yes      | Value between 1 and 100 |
+| `profile`   | string | no       | `spike` (default), `ignition`, `powerline`, or `broadband` |
 
 ### How it works
 
-The blanker runs on every IQ block (4096 samples) *before* the mixer stage in
-the DSP pipeline:
+The blanker runs on every IQ block *before* the mixer stage in the DSP pipeline,
+one sample at a time:
 
-1. For each sample, compute magnitude² (`re² + im²`).
-2. Compare against `threshold² × mean_sq` (the exponentially-smoothed running
-   mean of magnitude²).
-3. If the sample exceeds the threshold, replace it with the previous clean
-   sample.
-4. Otherwise, update the running mean with smoothing factor α = 1/128 and store
-   the sample as the last clean value.
+1. Emit the sample from the look-ahead delay line and ingest the fresh one.
+2. Compute the fresh sample's power (`re² + im²`) and compare it against
+   `threshold² × noise_floor`.
+3. If it exceeds the threshold, hold the gate closed for the profile's blank
+   window; the fresh sample reaches the output a few samples later, by which
+   time the gate has fully ramped to zero — so the leading edge is removed.
+4. Otherwise, update the noise-floor estimate (skipped while blanking) and let
+   the gate ramp back open.
 
 Because the blanker operates on raw IQ before frequency translation, it removes
 impulse noise across the entire captured bandwidth regardless of the tuned