refactor(engine): type the captured child-boundary stack depth

[matthewevans]

Close FrameVec::slot_at_captured_depth, the last bare-usize door into
resolution-frame addressing, deferred by #7489.

ChildStackDepth is an opaque newtype whose tuple field is private to
frame_vec, so resolution -- its parent module -- cannot construct one.
A usize from a scan still compiles and now has nowhere to go: the only
mint is FrameVec::capture_depth, which reads the stack's own length.

Threads the type through 15 capture origins, 8 GameState wrappers, 6
ResolutionStack doors, 5 park_* helper params, 9 Ord comparison reads and
3 ResolutionStackError fields. No serialized type is affected.

The structural guard gains four conditions built on a balanced-paren
signature extractor, so a second bare-usize parameter in frame_vec.rs
fails CI even when it hides behind a closure or a generic parameter.

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Rebase note. Cherry-picked onto an origin/main that had advanced 9 commits past the reviewed base (a8244e73). One conflict, in change_zone.rs, where the reviewed commit changed the depth-capture line and upstream (#7496) changed the adjacent call line to execute_zone_move_with_terminal_and_controller. Resolved keep-both. The resolved diff's added/removed lines are byte-identical to the reviewed diff (+337/-116, 11 files); only surrounding context moved. The boundary guard passes on the rebased tree.

[github-actions]

Generated for head 57c51f022678004b643aa5863597cb825c7913e2.

Parse changes introduced by this PR

✓ No card-parse changes detected.

[coderabbitai]

Caution

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