[perf](trx-backend-soapysdr): replace FIR with FFT overlap-save via rustfft

Replace the per-sample ring-buffer FIR convolution with block-level
overlap-save convolution using rustfft. For a block of M samples and
N taps the old approach costs O(N·M); the new one costs O(M log M),
with rustfft using SIMD (AVX2/SSE4) internally.

Key changes:
- Add rustfft = "6" dependency
- Add BlockFirFilter: overlap-save filter with pre-computed H(f) and
  a single forward+inverse FFT pair per block (no per-sample multiply)
- ChannelDsp.process_block() now:
  1. Batch-mixes entire block to baseband in one vectorisable loop
  2. Applies BlockFirFilter to I and Q (one FFT pair each)
  3. Decimates and demodulates as before
- Keep the old FirFilter for unit tests (sample-by-sample interface)
- Add BlockFirFilter unit tests (DC passthrough, length preservation)
- IQ_BLOCK_SIZE promoted to pub const for use in filter sizing

For the default config (4096-sample blocks, 64 taps, decim=40):
  Old: ~262144 multiply-adds per FIR × 2 components = ~524k per block
  New: ~2 × (3 × 8192 × log2(8192)) ops, all SIMD-vectorised by rustfft

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Stan Grams <sjg@haxx.space>

Cargo.lock

@@ -1461,6 +1461,15 @@ dependencies = [
  "syn",
 ]
 
+[[package]]
+name = "num-integer"
+version = "0.1.46"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7969661fd2958a5cb096e56c8e1ad0444ac2bbcd0061bd28660485a44879858f"
+dependencies = [
+ "num-traits",
+]
+
 [[package]]
 name = "num-traits"
 version = "0.2.19"
@@ -1619,6 +1628,15 @@ dependencies = [
  "zerocopy",
 ]
 
+[[package]]
+name = "primal-check"
+version = "0.3.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "dc0d895b311e3af9902528fbb8f928688abbd95872819320517cc24ca6b2bd08"
+dependencies = [
+ "num-integer",
+]
+
 [[package]]
 name = "proc-macro-crate"
 version = "3.4.0"
@@ -1787,6 +1805,20 @@ dependencies = [
  "semver",
 ]
 
+[[package]]
+name = "rustfft"
+version = "6.4.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "21db5f9893e91f41798c88680037dba611ca6674703c1a18601b01a72c8adb89"
+dependencies = [
+ "num-complex",
+ "num-integer",
+ "num-traits",
+ "primal-check",
+ "strength_reduce",
+ "transpose",
+]
+
 [[package]]
 name = "rustversion"
 version = "1.0.22"
@@ -2003,6 +2035,12 @@ version = "1.2.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "6ce2be8dc25455e1f91df71bfa12ad37d7af1092ae736f3a6cd0e37bc7810596"
 
+[[package]]
+name = "strength_reduce"
+version = "0.2.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "fe895eb47f22e2ddd4dabc02bce419d2e643c8e3b585c78158b349195bc24d82"
+
 [[package]]
 name = "strsim"
 version = "0.11.1"
@@ -2317,6 +2355,16 @@ dependencies = [
  "tracing-log",
 ]
 
+[[package]]
+name = "transpose"
+version = "0.2.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1ad61aed86bc3faea4300c7aee358b4c6d0c8d6ccc36524c96e4c92ccf26e77e"
+dependencies = [
+ "num-integer",
+ "strength_reduce",
+]
+
 [[package]]
 name = "trx-app"
 version = "0.1.0"
@@ -2378,6 +2426,7 @@ name = "trx-backend-soapysdr"
 version = "0.1.0"
 dependencies = [
  "num-complex",
+ "rustfft",
  "serde",
  "soapysdr",
  "tokio",