fix: clarify rust-hybrid fallback messaging for non-Rust-owned files (Issue #680)

[jununfly]

Issue

Fixes #680 — CLI says '30 TypeScript fallback files' but actual is 30 YAML files indexed via the TypeScript fallback path. The label conflated fallback implementation (TypeScript) with source file language.

Root Cause

FALLBACK_REASON_LABELS['language-level-typescript-fallback'] was set to 'TypeScript fallback files', which reads as '30 TypeScript source files' instead of '30 non-Rust-owned files handled by the TypeScript fallback path'.

Fix: Distinguish reason from source language (Q1=B, Q2=B)

Three CLI print locations updated:

1. zcodegraph index summary — clack.log.warn reworded to 'non-Rust-owned files via TypeScript fallback'
2. zcodegraph status — fallbackSummaryFromHybridMetadata() now extracts fallbackByLanguage from DB metadata
3. zcodegraph doctor — buildDiagnosticBundleSummary() now passes fallbackByLanguage from aggregateTaxonomy

New output format:

Top fallback reasons:
  30 non-Rust-owned files via TypeScript fallback
Fallback by source language: yaml (30)

Data pipeline (Q3=A):
fallbackByLanguage flows from planRustHybridAssignments() → typescriptFallbackAppend profile → RustHybridFallbackSummary → format layer.

Storage message unchanged (Q4=A): fallbackMessage() in rust-hybrid-contract.ts kept as-is for backward compatibility. Diagnostic bundle already has structured fallbackByLanguage field.

TDD Vertical Slices

Slice	Tests	Implementation
1-1	4 tests: label match, language breakdown line, multi-language, absent case	fallback-summary.ts
1-2	(covered by 1-1)	zcodegraph.ts — fallbackSummaryFromHybridMetadata
1-3	(covered by 1-1)	zcodegraph.ts — profile construction (3 sites)
1-4	(covered by CLI fallback tests)	zcodegraph.ts — clack.log.warn
1-5	(covered by 1-1)	diagnostics/index.ts — buildDiagnosticBundleSummary

Files Changed

• src/diagnostics/fallback-summary.ts — label, type, format function
• src/bin/zcodegraph.ts — metadata extraction, profile (3 sites), warn message
• src/diagnostics/index.ts — diagnostic bundle summary
• __tests__/issue-680-fallback-messaging.test.ts — 4 new TDD tests
• __tests__/rust-hybrid-fallback-summary.test.ts — update label assertion
• __tests__/rust-index-engine-cli-fallback.test.ts — update label assertions

Tests

npx vitest run __tests__/issue-680-fallback-messaging.test.ts __tests__/rust-hybrid-fallback-summary.test.ts — 8/8 pass
npx tsc --noEmit — zero errors