Add ESM named fallback reason samples to Rust profile

[jununfly]

This was generated by AI during triage.

Parent

• Plan: docs/plans/2026-06-21-rust-hybrid-esm-named-binding-fallback-diagnostics-map.md
• Resolver migration tracker: Plan: Rust-hybrid resolver migration decision #296
• Architecture/performance PRD tracker: PRD: Rust-hybrid architecture and performance optimization #295
• Optimization tracker: Post-PRD optimization tracker for Rust indexing performance #165

What to build

Extend Rust-hybrid profile diagnostics so ESM named import/export fallback is no longer exposed only as an aggregate count. The profile should include bounded reason counts and bounded privacy-safe samples that explain why named binding resolution fell back.

This is diagnostics-only. Do not change resolver behavior, graph edges, package/default/namespace semantics, or public user-facing surfaces.

Acceptance criteria

• Rust profile includes bounded ESM named fallback reason counts.
• Rust profile includes bounded ESM named fallback samples and a sample cap/truncation contract.
• Samples distinguish missing/ambiguous import edge, type-only boundary, direct export candidate failures, one-hop re-export candidate failures, and package/runtime or unsupported boundaries.
• Samples use only privacy-safe fields and do not include source snippets, source lines, export-list text, or candidate name arrays.
• Existing ESM named import/export success behavior still passes.

Blocked by

None - can start immediately.

[jununfly]

This was generated by AI during triage.

Closing as implemented in aaba106. Rust-hybrid profile artifacts now expose bounded esmNamedImportExportFallbackSampleCounts, esmNamedImportExportFallbackSamples, and esmNamedImportExportFallbackSampleCap fields with privacy-safe ESM named fallback reasons. Existing ESM named success behavior remains covered.