Cache per-asset constraint map in graph view

[VPDPersonal]

Summary

• Cache the per-asset managed-reference constraint map in SerializeReferenceGraphView so opening the "Fix Missing" / "Change" picker no longer re-runs BuildConstraintMap (a LoadAllAssetsAtPath + full SerializedObject walk) on every card click.
• Mirror the existing SerializeReferenceProjectView pattern: an instance _constraintCache keyed by asset path, populated lazily in ResolveConstraint (now an instance method) and cleared at the top of Rescan (which every apply path funnels through) so a fix's rewritten YAML is re-read.

Notes for review

• _constraintCache.Clear() lives at the start of Rescan; ApplyFix, ClearReference, ApplyLive, and ClearOrphan all end by calling Rescan, so the cache can never serve a map staled by an in-view edit.
• Keyed by asset path (not a single map) so a multi-document asset still shares one map across its cards while staying correct if the target ever changes.

Linked issues

Refs #49 - addresses review finding #49 (comment)