fix(bestfit): drop saved pair sets whose canonical frame changed

Saved best-fit pairs are in the drawing's canonical frame, but the canonical
angle is recomputed on load and was not saved with the set. When that
computation changes (RotatingCalipers checking every hull edge, marks no
longer counted as material), old pairs were replayed in a turned frame and
could overlap.

Each saved set now records canonicalAngle, and BestFitCacheStore.Import
skips sets whose angle no longer matches the recomputed one (1e-9 rad,
modulo a full turn). Sets from older files have no angle and are dropped;
their pairs are recomputed when next needed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/nest-file-format.md

@@ -149,12 +149,21 @@ it now explicitly refreshes only the edited drawing's layout colors.
 
 Optional `bestfits/bestfit-N` entries hold drawing N's cached best-fit pair
 results: a JSON array of sets, each with `plateWidth`, `plateHeight` and
-`spacing` (the plate size and part spacing it was filtered for) and `results`.
-Each result has `part1Rotation`, `part2Rotation`, `part2OffsetX`,
-`part2OffsetY`, `strategyType`, `testNumber`, `candidateSpacing`,
-`rotatedArea`, `boundingWidth`, `boundingHeight`, `optimalRotation`, `keep`,
-`reason`, `trueArea` and `hullAngles`. Angles are radians; pair geometry is in
-the drawing's canonical frame.
+`spacing` (the plate size and part spacing it was filtered for),
+`canonicalAngle` and `results`. Each result has `part1Rotation`,
+`part2Rotation`, `part2OffsetX`, `part2OffsetY`, `strategyType`, `testNumber`,
+`candidateSpacing`, `rotatedArea`, `boundingWidth`, `boundingHeight`,
+`optimalRotation`, `keep`, `reason`, `trueArea` and `hullAngles`. Angles are
+radians; pair geometry is in the drawing's canonical frame.
+
+The canonical angle is not saved with the drawing: readers recompute it from
+the program, so a change to that computation turns the frame of drawings saved
+before it. `canonicalAngle` records the drawing's canonical angle when the set
+was saved, and `BestFitCacheStore` drops a set on load unless it matches the
+recomputed angle (within 1e-9 radians, modulo a full turn), since pairs replayed
+in a turned frame can overlap. Sets from files saved before the field existed
+have no angle and are dropped the same way; their pairs are recomputed when
+next needed.
 
 The entries are only a cache: readers may ignore them and writers may omit
 them, at the cost of recomputing pairs. Writers save only sets whose