Rewrite sras_average.py for v6/v7, memory-bounded for multi-hundred-GB scans

The old tool only understood the legacy uniform-geometry header and had no
bound on peak RAM, making it unusable on the ragged v6/v7 files the scanner
now produces at up to ~560GB. Reads via SrasFile's memmap and writes in
row-chunks sized to a memory budget (SRAS_MEM_BUDGET_MB), stages to .part and
os.replace()s per scan_format.md's atomic-write requirement, always writes
plain v6 (a v7 cache tail is frame-indexed and invalid after averaging), and
divides laser_freq_hz by N so x_axis_mm() stays correct after the X axis gets
spatially binned.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tools/make_test_sras.py

@@ -266,10 +266,7 @@ def write_rotating(path: Path, n_angles: int = 5, samples_per_frame: int = 4,
 def write_legacy(path: Path, version: int = 4, n_angles: int = 2,
                  n_rows: int = 4, n_frames: int = 10,
                  samples_per_frame: int = 32, seed: int = 0) -> dict:
-    """Write a v2/v3/v4 file: uniform geometry, one flat waveform block.
-
-    Used to exercise sras_average.py, which only handles the legacy formats.
-    """
+    """Write a v2/v3/v4 file: uniform geometry, one flat waveform block."""
     rng = np.random.default_rng(seed)
     n_ch, bps = 3, 1