feat: support $dynamicRef\mathrm{/}dynamicRef\; /$dynamicAnchor (JSON Schema 2020-12)  (#337)

• feat: resolve $dynamicRefasastaticdynamicRef\; as\; a\; static$ref and register $dynamicAnchor (2020-12)

Implement the first half of JSON Schema 2020-12 $dynamicRef\mathrm{/}dynamicRef/$dynamicAnchor support:

• Register $dynamicAnchornamesasordinaryanchorssoareferencecanresolvetothem,exactlylikedynamicAnchor\; names\; as\; ordinary\; anchors\; so\; a\; reference\; can\; resolve\; to\; them,\; exactly\; like$anchor.
• Resolve $dynamicReftoitsinitial(lexically-static)targetandvalidatetheinstanceagainstit,sodynamicRef\; to\; its\; initial\; (lexically-static)\; target\; and\; validate\; the\; instance\; against\; it,\; so$dynamicRef behaves like a plain $reftothelocationitinitiallypointsat\mathrm{.}Thetargetiskeptashidden,non-enumerablemetadata(ref\; to\; the\; location\; it\; initially\; points\; at.\; The\; target\; is\; kept\; as\; hidden,\; non-enumerable\; metadata\; ($dynamicRefTarget) so sibling keywords on the reference are preserved.
• Remove the previous ‘unsupported feature’ warning for $dynamicRef\mathrm{/}dynamicRef/$dynamicAnchor and update the corresponding tests.

True dynamic-scope (bookended) resolution is added in a follow-up commit; the 7 official test-suite cases that require it are temporarily skipped.

• feat: add dynamic-scope resolution for $dynamicRef (2020-12)

Complete JSON Schema 2020-12 $dynamicRef support with true dynamic-scope (“bookended”) resolution:

• Add a pre-pass (collectDynamicAnchors) that, from the original schema tree before $refmergingflattensresourceboundaries,buildsper-resourceanchormaps(ref\; merging\; flattens\; resource\; boundaries,\; builds\; per-resource\; anchor\; maps\; ($anchorMaps: dynamic-only and all-anchor) and records each $dynamicRe{f}^{\mathrm{\prime }}slexicalresource(indynamicRef\text{'}s\; lexical\; resource\; (in$dynamicRefInfo.scope).
• Resolve an internal $dynamicRef’s initial target within its own lexical resource so a sibling resource’s identically-named anchor cannot leak in.
• At validation time, apply the bookending rule: when the initial target is a $dynamicAnchorofthereferencedname,resolvetotheoutermostmatchingdynamicAnchor\; of\; the\; referenced\; name,\; resolve\; to\; the\; outermost\; matching$dynamicAnchor currently in the dynamic scope; otherwise behave like a plain $ref.
• Apply $dynamicRef alongside any sibling keywords on the same node, rather than treating it as a redirect that replaces them.
• Propagate the hidden $dynamicRefmetadatathroughschemamergingsoadynamicRef\; metadata\; through\; schema\; merging\; so\; a$dynamicRef living directly on a $ref’d resource still resolves.
• Ignore $dynamicRef as an unknown keyword in drafts before 2020-12.
• Store the internal, non-enumerable metadata in two consolidated records (DynamicRefInfo and AnchorMaps) rather than five separate hidden fields.

All 32 official dynamicRef test-suite cases now pass; the temporary skips added in the previous commit are removed.

• test: cover $dynamicRef\mathrm{/}dynamicRef/$dynamicAnchor edge cases (2020-12)

Add project-owned tests for the gaps found in the $dynamicRef coverage review, so the behavior no longer relies solely on the vendored official suite:

• a sub-schema shared by reference is anchor-collected once (shared-node guard) and raises no spurious duplicate-anchor error
• a duplicate $dynamicAnchor within one resource is reported as a schema resolution error
• genuine dynamic-scope resolution: an outer $dynamicAnchor overrides a base resource’s default (extendible pattern)
• a $dynamicReftoaplaindynamicRef\; to\; a\; plain$anchor (not bookended) behaves like a plain $ref
• a $dynamicRefwithaJSON-pointerfragmentbehaveslikeaplaindynamicRef\; with\; a\; JSON-pointer\; fragment\; behaves\; like\; a\; plain$ref
• a $dynamicRef resolved through an external schema
• $dynamicRef is ignored as an unknown keyword in draft 2019-09
• completion and hover resolve through a $dynamicRef node

• fix: extend $dynamicRef dynamic scope across external documents (2020-12)

The dynamic-scope pre-pass (collectDynamicAnchors) ran only over the root document’s resources, so a $dynamicAnchorthatoverridesabasedefaultwashonoredonlywhenitlivedinthedocumentbeingvalidated\mathrm{.}Whentheextendingresourcelivedina\ast separate\ast documentreachedviadynamicAnchor\; that\; overrides\; a\; base\; default\; was\; honored\; only\; when\; it\; lived\; in\; the\; document\; being\; validated.\; When\; the\; extending\; resource\; lived\; in\; a\; *separate*\; document\; reached\; via$ref, its $dynamicAnchor never joined the dynamic scope and the override was lost.

• Collect each referenced document’s own anchor maps (once) during external link resolution, before $refmergingflattensresourceboundaries,soitsref\; merging\; flattens\; resource\; boundaries,\; so\; its$dynamicAnchor declarations can participate in the dynamic scope and its $dynamicRef nodes get their lexical resource recorded.
• Fold a merged section’s anchor maps into the target during merge, with outermost-resource-wins precedence, so a $dynamicAnchordefinedinanexternal(ordynamicAnchor\; defined\; in\; an\; external\; (or$ref’d sub-) resource can override a base default while the base’s own entries keep priority.
• Derive the effective draft from each resource’s own $schemasoareferencedpre-2020-12documentisnotgivenschema\; so\; a\; referenced\; pre-2020-12\; document\; is\; not\; given$dynamicAnchor semantics.

Tests cover the cross-document override for both an internal-fragment (dynamicRef "#name") and a non-fragment URI (dynamicRef “other#name”) starting point. Each case uses a fresh service and a fresh schema object: in-place $ref resolution mutates the schema it is given, so reusing one object would let a prior validation leak anchor state and mask the gap.