Simplify pass on top of #2070 fix:
- Rename pruneStaleSubtreeIds → pruneStaleKeys, generalize to
Map<string, T> so the same shape can absorb other keyed-by-node-id
caches (ProvisioningTimeout.tsx tracking map is the obvious next
caller — left as a follow-up to keep this PR scoped).
- Trim the helper docstring to remove implementation-detail rot
(O(map_size), cadence claims). The ref-block comment carries the
rationale where it actually matters (at the call site).
- Add identity-preservation test: survivors must keep their original
Set reference. Guards against a future "rebuild instead of delete"
regression that would silently invalidate downstream === checks.
No behaviour change.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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