[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>

src/trx-server/trx-backend/trx-backend-soapysdr/Cargo.toml

@@ -14,4 +14,5 @@ tokio = { workspace = true, features = ["sync", "rt"] }
 serde = { workspace = true }
 tracing = { workspace = true }
 num-complex = "0.4"
+rustfft = "6"
 soapysdr = "0.3"