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_viewer/common.py

@@ -2,8 +2,8 @@
 
 from PyQt6.QtCore import Qt
 from PyQt6.QtWidgets import (
-    QDoubleSpinBox, QFormLayout, QFrame, QGroupBox, QLabel, QScrollArea,
-    QSizePolicy, QVBoxLayout, QWidget,
+    QComboBox, QDoubleSpinBox, QFormLayout, QFrame, QGroupBox, QLabel,
+    QScrollArea, QSizePolicy, QVBoxLayout, QWidget,
 )
 
 from sras_format import CH1_IDX, CH3_IDX, CH4_IDX
@@ -85,6 +85,29 @@ def _make_dspin(lo: float, hi: float, decimals: int, *, suffix: str = "",
     return spin
 
 
+def _combo(items=(), *, min_chars: int = 10) -> QComboBox:
+    """A combo box whose size hint does not depend on its longest entry.
+
+    By default a QComboBox asks for enough width to show its widest item. These
+    hold descriptive phrases, and the side panels are fixed-width — in a scroll
+    area with the horizontal scrollbar off (`_scroll_panel`) an unconstrained
+    hint pushes the inner widget past the panel and everything on the right,
+    including the hint text, is silently clipped instead of scrolling.
+
+    *items* is a sequence of (label, data) pairs, or of plain labels.
+    """
+    combo = QComboBox()
+    combo.setSizeAdjustPolicy(
+        QComboBox.SizeAdjustPolicy.AdjustToMinimumContentsLengthWithIcon)
+    combo.setMinimumContentsLength(min_chars)
+    for item in items:
+        if isinstance(item, tuple):
+            combo.addItem(item[0], item[1])
+        else:
+            combo.addItem(item)
+    return combo
+
+
 def _axes_extent(x_axis, y_axis, dx: float, dy: float) -> list[float]:
     """Matplotlib imshow extent with half-pixel margins, Y flipped so row 0
     renders at the top."""