fix: add three-tier fallback health state (healthy/partial/degraded) for Issue #682

[jununfly]

Summary

Distinguish expected fallbacks (non-Rust-owned languages like YAML) from unexpected gaps (Rust-owned parse/extraction failures, missing files).

Problem

Issue #682 reported that indexing a TypeScript-heavy project (ZAgenticLoop) showed Fallback health: degraded for 58 YAML files — but these are expected fallbacks, not problems. The degraded label caused unnecessary user concern.

Solution

Three-tier fallback health state:

State	Condition
healthy	Zero fallbacks, zero gaps
partial	Only language-level-typescript-fallback (non-Rust-owned languages, successfully indexed)
degraded	Any rust-owned-* gap or language-level-fallback-missing-file

Key changes

• RustHybridFallbackState type: add 'partial'
• rustHybridFallbackStateFor(): three-tier logic checking taxonomy keys
• formatRustHybridFallbackHealthLines(): partial output with info line + language breakdown (no "need review")
• formatRustHybridFallbackDoctorHint(): partial returns hint without "Run diagnostic bundle"
• clack.log: conditional info (partial) / warn (degraded)
• statusFallbackDiagnostics: returns data for partial (empty doctorCommand)
• classifyGraphHealth: partial treated as healthy graph health
• diagnostics/index.ts: fallbackState override handles partial

Before (Issue #682 scenario)

Fallback health: degraded
The index is usable; fallback-degraded files or diagnostics are the only parts that need review.
Top fallback reasons:
  58 non-Rust-owned files via TypeScript fallback

After

Fallback health: partial
Index is complete; some files were indexed via TypeScript fallback for non-Rust-owned languages.
58 files indexed via TypeScript fallback (non-Rust-owned languages)
Fallback by source language: yaml (58)

Test plan

• 11 new TDD tests in issue-682-partial-fallback-state.test.ts
• 7 existing tests updated (rust-hybrid-fallback-summary, issue-680-messaging, rust-index-engine-cli-fallback)
• tsc --noEmit zero errors
• 19/19 unit tests pass
• CLI integration tests require cargo (not available in this environment)

Closes #682