Refactor cache validation and optimize alignment wizard incremental updates

- Extract cache mismatch logic into reusable cache_mismatch_reasons() function
  for cleaner validation and consistent miss reporting
- Expose memory_budget_bytes() for external callers (aligned exporter)
- Optimize rebuild_stack() with incremental layer updates when only one angle
  parameters change, avoiding full reprojection overhead
- Remove unused seed_per_angle parameter from alignment wizard
- Simplify cached_rf_image() DC masking with cleaner early exit logic
- Clean up internal state tracking with explicit origin caching

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

sras_format.py

@@ -254,6 +254,48 @@ class SrasFile:
         # be served from it — see sras_compute.cached_rf_image.
         self.precomputed_pad_factor: int = 1
 
+    def encoded_preambles(self) -> bytes:
+        """This file's Preamble Blocks section, as bytes a writer can emit.
+
+        v6/v7 files kept the on-disk span verbatim, which is both cheaper and
+        lossless; legacy files did not keep it, and a v2 file has no preambles
+        at all, so those are re-encoded from the parsed strings (empty ones for
+        v2). Empty is not a silent downgrade: _parse_preamble("") returns {} and
+        _set_calibration falls back to the hardcoded scope constants, which is
+        exactly the calibration a v2 file already gets, so mV values round-trip
+        unchanged.
+
+        Lives here rather than at each writer so the version fan-out sits next
+        to the parser that creates it, and no writer has to probe the object
+        to find out which shape it got.
+        """
+        raw = getattr(self, "preambles_raw", None)
+        if raw is not None:
+            return raw
+        out = bytearray()
+        for s in self.preambles or [""] * self.n_channels:
+            encoded = s.encode("utf-8")
+            out += struct.pack(">H", len(encoded)) + encoded
+        return bytes(out)
+
+    def encoded_background(self) -> bytes:
+        """This file's Background Block, as bytes a writer can emit.
+
+        When there is none (v2/v3), this is samples_per_frame zeros rather than
+        a zero-length block. Every consumer guards on `background is not None`
+        and then subtracts it from a (spf,)-shaped row, so a length-0 array
+        would broadcast-fail at the first background-subtracted FFT; zeros make
+        the subtraction a correct no-op instead.
+        """
+        raw = getattr(self, "background_raw", None)
+        if raw is not None:
+            return raw
+        if self.background is None:
+            samples = np.zeros(self.samples_per_frame, dtype=np.int8)
+        else:
+            samples = np.rint(self.background).astype(np.int8)
+        return struct.pack(">I", samples.size) + samples.tobytes()
+
     def cached_dc_mv(self, angle_idx: int, ch_idx: int) -> np.ndarray | None:
         """A stored DC image (already in mV) for (angle, channel), or None."""
         store = self.precomputed_dc3_mv if ch_idx == CH3_IDX else self.precomputed_dc4_mv
@@ -284,6 +326,15 @@ class SrasFile:
             self.scan_aborted = False
             self.n_angles_declared = n_angles
 
+            # Pre-v6 files carry no nominal ROI. Defined as None rather than
+            # left absent so the object's shape does not depend on its version
+            # and writers can ask instead of probing with hasattr.
+            self.x_start_nominal_mm = None
+            self.y_start_nominal_mm = None
+            self.x_delta_nominal_mm = None
+            self.y_delta_nominal_mm = None
+            self.row_spacing_mm = None
+
             angles = np.frombuffer(f.read(n_angles * 4), dtype=">f4").astype(np.float32)
             y_pos = np.frombuffer(f.read(n_rows * 4), dtype=">f4").astype(np.float32)