feat: resolve $dynamicRef against the dynamic scope

[aqeelat]

Part of #359.

Implements the "dynamic-scope-aware dereferencing" item from the #359 task list. locallyDereferenced() / JSONSchema.dereferenced(in:) now resolve $dynamicRef against the dynamic scope (the outermost in-scope $dynamicAnchor wins, per JSON Schema 2020-12 §7.7), instead of throwing on every $dynamicRef.

Behavior

• Non-recursive $dynamicRef (e.g. the JSON Schema "generics" pattern — $dynamicRef "#T" → a leaf $defs entry): the target is inlined.
• Recursive or unresolvable $dynamicRef: throws (ReferenceCycleError / GenericError), consistent with how recursive static $ref already fails and with the Dereferenced... invariant that a DereferencedJSONSchema contains no references.
• The dynamic scope is reconstructed from the document's static structure: seeded from each schema's own $dynamicAnchor (and $defs-declared anchors, first-wins/outermost-wins) and threaded through subschemas and across $ref boundaries.

Notes

• Raw-AST consumers (most generators, e.g. swift-openapi-generator) are unaffected — .dynamicReference on JSONSchema is unchanged; only locallyDereferenced() behavior changes (strictly an improvement: previously it threw on all $dynamicRef; now only recursive/unresolvable ones).
• Edits the same migration-guide section as feat: externally dereference $dynamicRef #503 (external deref); trivial doc reconcile on merge.

Testing

• New deref tests: generics-inline, recursive-throws, unresolvable-throws, non-anchor-throws, optional+described propagation, sibling dynamic-ref independence, scope-propagates-across-$ref, $ref→$dynamicRef name propagation, optional $ref property.
• Full suite: 2165 tests, 0 failures (2157 prior + 8 new; no regressions — the .reference refactor is behavior-preserving).
• 100% region coverage on the new code (llvm-cov-verified: 0 uncovered regions in the scope-aware _dereferenced + the ArrayContext/ObjectContext threading).

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This PR was drafted with assistance from AI tooling. The submitter has reviewed and validated the contents prior to submission.