Implement row-averaged FFT feature for same-row SNR cleanup

Add row-averaged FFT feature with configurable window size (row_avg_n
parameter) for improved signal-to-noise ratio on noisy scans. Includes:

- Gaussian-weighted same-row neighbor averaging (never crosses rows)
- Masked/renormalized convolution handling for edge cases and masked samples
- Cache format v2 with row_avg_n tracking to prevent silent cache mismatches
- GUI dialog option for row-average window configuration
- Comprehensive tests validating kernel properties, background subtraction
  invariance, and cache dispatch

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

scan_format.md

@@ -220,7 +220,7 @@ actions.
 | Offset | Size | Type | Field | Description |
 |--------|------|------|-------|-------------|
 | 0 | 4 | `char[4]` | `cach_magic` | `CACH` (ASCII). Missing/wrong magic → treat file as having no cache. |
-| 4 | 1 | `u8` | `cach_version` | Cache format version. Currently `1`. Readers must treat the file as uncached if this is not a version they understand (unlike v5's `PREC` section, which read but never validated its version byte). |
+| 4 | 1 | `u8` | `cach_version` | Cache format version. Currently `2`; readers also accept `1` (a `1` tail predates row-averaged FFT caching — see the `SFFT` block below and [CACH tail version history](#cach-tail-version-history)). Any other value → treat the file as uncached (unlike v5's `PREC` section, which read but never validated its version byte). |
 | 5 | 1 | `u8` | `block_flags` | Bit 0 = DC block (`SDCB`) follows. Bit 1 = FFT block (`SFFT`) follows, immediately after the DC block if both are present. Bits 2–7 reserved, must be zero on write. |
 
 ### DC block `SDCB` (present iff `block_flags & 0x01`)
@@ -249,13 +249,20 @@ value, same as v5's `PREC` section.
 
 ### FFT block `SFFT` (present iff `block_flags & 0x02`)
 
-7-byte block header, format `">4sBH"`:
+Block header layout depends on `cach_version`:
+
+- **`cach_version` 1**: 7 bytes, format `">4sBH"` — magic, flags, n_stored.
+- **`cach_version` 2**: 8 bytes, format `">4sBHB"` — magic, flags, n_stored,
+  `row_avg_n`. Always written by current code; a `cach_version` 1 tail (no
+  trailing byte) is still read, with `row_avg_n` taken as `0` for every
+  entry it stores.
 
 | Offset (rel) | Size | Type | Field | Description |
 |--------------|------|------|-------|-------------|
 | 0 | 4 | `char[4]` | `magic` | `SFFT` |
-| 4 | 1 | `u8` | `flags` | Bit 0 = `bg_sub_applied` — background waveform was subtracted from CH1 before the FFT when these images were computed. Bits 1–7 reserved. |
+| 4 | 1 | `u8` | `flags` | Bit 0 = `bg_sub_applied` — background waveform was subtracted from CH1 before the FFT when these images were computed. Bit 1 = `row_averaged` — `peak_freq_mhz` came from same-row, distance-weighted averaged CH1 waveforms rather than raw per-pixel ones; `row_avg_n` (below) is the neighbor half-width used. Bits 2–7 reserved. |
 | 5 | 2 | `u16` | `n_stored` | Number of angle entries that follow |
+| 7 | 1 | `u8` | `row_avg_n` | *`cach_version` 2 only.* Same-row neighbor half-width, in pixels, that `peak_freq_mhz` was averaged over before its FFT; `0` = raw (unaveraged). Meaningful only when `flags` bit 1 is set — a `cach_version` 1 tail has no such byte and is always `row_avg_n = 0`. |
 
 followed by `n_stored` entries, each:
 
@@ -264,9 +271,10 @@ u16                          angle_idx      — index into the angle table (0-ba
 f32[n_rows[angle_idx] × n_frames[angle_idx]]  peak_freq_mhz  — CH1 FFT peak frequency, MHz, row-major
 ```
 
-**`peak_freq_mhz`** is computed without any DC-threshold masking (i.e. the
-FFT is run on every pixel unconditionally, same as v5's `PREC` convention).
-Readers apply the DC4 threshold at display time:
+**`peak_freq_mhz`** for a raw store (`row_avg_n == 0`) is computed without
+any DC-threshold masking (i.e. the FFT is run on every pixel
+unconditionally, same as v5's `PREC` convention). Readers apply the DC4
+threshold at display time:
 
 ```
 pixel is valid  ⟺  dc4_mv[r][f]  ≥  threshold_mv
@@ -276,10 +284,22 @@ display_value   =   peak_freq_mhz[r][f]  if valid, else 0
 using the DC4 image from the DC block if that angle is also cached there,
 else computed on demand.
 
+For a row-averaged store (`row_avg_n > 0`), the DC4 threshold is applied
+*during* the store — a pixel below threshold is left at `0` and never
+contributes to any neighbor's average — since neighbor validity can't be
+deferred to display time the way plain masking can. The threshold value
+itself is not recorded, only that averaging happened and at what window
+size. Readers still apply their own live DC4 threshold at display time
+exactly as for a raw store, using whatever mask they currently have.
+
 Readers must fall back to real-time FFT computation (ignoring stored
 `peak_freq_mhz`) under the same conditions as v5's PREC fast path: time-domain
-gating is active, zero-padding (`n_fft ≠ samples_per_frame`) is requested, or
-the reader's background-subtraction setting doesn't match `flags.bg_sub_applied`.
+gating is active, zero-padding (`n_fft ≠ samples_per_frame`) is requested,
+the reader's background-subtraction setting doesn't match
+`flags.bg_sub_applied`, or the reader's requested `row_avg_n` doesn't match
+the stored value exactly — a raw request must never be served a
+row-averaged store, or vice versa, and a request at one window size must
+never be served a store at another.
 
 ### In-place write ordering
 
@@ -294,6 +314,17 @@ interrupted write leaves harmless trailing bytes rather than a corrupt file,
 and the next successful write overwrites them via the same deterministic
 `cache_offset`.
 
+### CACH tail version history
+
+Distinct from the outer `.sras` file `version` byte (top of this document),
+which has stayed `7` since the Cache Tail was introduced — this is the inner
+`cach_version` byte inside the `CACH` header itself.
+
+| cach_version | Change |
+|--------------|--------|
+| 1 | Initial Cache Tail: `SDCB` (DC) and `SFFT` (FFT, 7-byte header) blocks. |
+| 2 | `SFFT` header grows one byte, `row_avg_n` — the same-row neighbor half-width the stored `peak_freq_mhz` was averaged over before its FFT, `0` = raw. Readers still accept a `cach_version` 1 tail, treated as `row_avg_n = 0` for every angle it stores, so files cached before this change keep working without a recompute. |
+
 ---
 
 ## Acquisition Settings (fixed by sc3_aui_app.py)