fix: harden document fallback paths

[lzehrung]

Summary

• harden markdown document-link extraction against malformed and nested link/image syntax without creating false edges
• enforce native:on availability consistently across graph-only, cached, and symbol/detail fallback paths
• add regressions covering malformed markdown, graph-only docs, and required-native behavior

Testing

• npm run test:fast tests/document-links.test.ts tests/fallback-import-extraction.test.ts tests/agent-orient.test.ts tests/native-fallback-contract.test.ts

[Copilot]

Pull request overview

This PR hardens document-link extraction and native tree-sitter fallback behavior so malformed/nested markdown link syntax doesn’t create false graph edges, and so native: "on" is enforced consistently across graph-only parsing, cached paths, and symbol/detail fallback paths.

Changes:

• Refactors markdown reference/inline link parsing to be more robust against malformed/nested constructs and bounded rescans.
• Introduces and wires assertNativeRequiredAvailable(...) to ensure required-native failures are not suppressed (including graph-only + cached flows).
• Adds regression tests for malformed markdown inputs, graph-only document behavior, and required-native enforcement.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tests/native-fallback-contract.test.ts	Adds regression coverage ensuring required-native errors propagate through locals enrichment and detailed symbol graph building.
tests/fallback-import-extraction.test.ts	Ensures graph-only documents don’t produce native-query reporting, while still honoring native: "on" requirements.
tests/document-links.test.ts	Expands markdown document-link regression suite for malformed/nested syntax and scan-boundary behaviors.
src/native/treeSitterNative.ts	Re-exports assertNativeRequiredAvailable from the native runtime API surface.
src/native/runtime.ts	Adds assertNativeRequiredAvailable helper to consistently enforce native: "on" availability checks.
src/indexer/parse-context.ts	Avoids native syntax-tree execution for graph-only languages while still enforcing required-native in prepare.
src/indexer/locals-and-exports.ts	Enforces required-native early and ensures required-native failures aren’t swallowed by fallback try/catch blocks.
src/indexer/build-workers.ts	Prevents graph-only files from going through native worker extraction/reporting paths.
src/indexer/build-index.ts	Enforces required-native at build start and forwards native mode into edge collection options consistently.
src/graphs/symbol-graph-detailed.ts	Enforces required-native upfront and ensures required-native errors propagate out of detailed-edge construction.
src/graph-edge-collector.ts	Enforces required-native even when serving cached edges and avoids native compact-import execution for graph-only languages.
src/documentLinks/markdown.ts	Reworks markdown reference definition/link scanning to handle malformed/nested inputs and avoid false positives.

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