doctor bundle: aggregate repeated identical per-file parser diagnostics (Vue/YAML fallback)

[jununfly]

Summary

Follow-up to #681 (closed via 38e3657). The three-tier fallback health state (ZJ-0007) resolved the degraded misreporting for Vue/YAML TypeScript fallback, but one acceptance criterion from #681 remains unimplemented: repeated identical per-file parser diagnostics are still emitted one-by-one in the doctor bundle instead of being summarized into an aggregate finding.

Current behavior

Running zcodegraph doctor --engine rust-hybrid --bundle --last-run on a Vue 3 + TypeScript monorepo where fallback succeeds produces per-file-diagnostics.json containing many identical entries, e.g.:

{
  "extension": ".vue",
  "language": "vue",
  "code": "parser_error",
  "severity": "error",
  "message": "Failed to get parser for language: typescript"
}

One entry per affected file (136 in the original report), even though the aggregate fallback health correctly reports partial and no user action is required.

Expected behavior

• Repeated same-code/same-message per-file diagnostics for the same language are collapsed into one aggregate finding with a count (e.g. vue: 136 files — "Failed to get parser for language: typescript" (parser_error)).
• The per-file detail remains available (replay-safe, path-hash-only) but is referenced from the aggregate finding rather than listed ungrouped.
• The bundle summary includes the aggregate findings when fallbackState is partial, so the diagnostics that do exist do not read as more severe than the aggregate outcome.

Acceptance criteria

• per-file-diagnostics.json (or a sibling aggregate file) groups identical code + message diagnostics per language with counts.
• doctor --bundle summary output surfaces aggregate findings for partial runs.
• Grouping preserves the replay-safe, path-hash-only contract (no source code or plaintext paths).
• Existing fallback-summary and diagnostics tests updated/extended to cover the grouping.

Context

• Original report: rust-hybrid doctor reports degraded fallback for Vue/YAML TypeScript fallback despite usable index #681
• Fix for the health-state semantics: 38e3657, design in docs/zj-adr/ZJ-0007-three-tier-fallback-health-state.md