feat(search): add fuzzy search with fzy-inspired scoring and position tracking

[danlylois-code]

Summary / 摘要

Add full fuzzy search support to the CodeMirror 6 search plugin: a 3-layer architecture spanning regex-bridge UI integration, fzy-inspired relevance scoring, and backtracking-based character position tracking. 104 tests pass, 0 lint errors.

Motivation / 背景与动机

• Roadmap: P2「模糊搜索」listed in ROADMAP.zh.md
• Users typing fuzzy queries (e.g. "fb" → "floatboat") get no matches with exact/regex search
• Goal: position-weighted relevance ranking so best matches appear first

Changes / 变更

packages/plugin-search:

• New interfaces: ScoredSearchMatch, DetailedFuzzyMatch
• New option: SearchOptions.fuzzy (boolean)
• Core engine: fuzzyToPattern() → regex bridge, computeFuzzyScore() → fzy-inspired scoring, findAllFuzzyMatchesDetailed() → backtracking matcher
• Public APIs: findScoredFuzzyMatches() (ranked), findDetailedFuzzyMatches() (per-char positions)
• UI: fuzzy toggle checkbox in search panel
• Safety: 200-char length guard (ReDoS protection), 10K result cap

Architecture (3-Layer Design) / 架构设计（3层设计）

┌─────────────────────────────────────────────────┐
│ Layer 0: Regex Bridge (CM6 Integration)          │
│   fuzzyToPattern() → /f.*?t/gi → SearchQuery     │
│   Zero changes to CM6 internals                   │
├─────────────────────────────────────────────────┤
│ Layer A: Scored Search API                       │
│   findScoredFuzzyMatches() → ScoredSearchMatch[] │
│   fzy-inspired: prefix bonus, consecutivity,     │
│   word-boundary bonus, gap penalty                │
├─────────────────────────────────────────────────┤
│ Layer B: Detailed Position Tracking              │
│   findDetailedFuzzyMatches() → DetailedFuzzyMatch│
│   matchedIndices[i] = doc offset of query[i]     │
│   Enables per-character highlight decorations    │
└─────────────────────────────────────────────────┘

Design Decisions / 设计决策

Decision	Rationale
Regex bridge instead of extending SearchQuery	SearchQuery is frozen CM6 internal; regex conversion feeds existing pipeline with zero fork
Non-greedy .*? between query chars	Finds shortest span containing all chars; greedy .* would span to last match
Greedy completion in backtracking matcher	O(n) per start position vs exponential backtracking; leftmost-shortest matches regex bridge semantics
Separate Scored and Detailed APIs	Interface segregation — most consumers skip matchedIndices allocation; upgrading is a type change
Fuzzy checkbox reset on updateQuery()	SearchQuery has no fuzzy field; external updates carry no fuzzy intent — prevents stale UI state

Testing / 测试

• pnpm test passes — 104 tests, 0 failures
• Affected packages build (pnpm build) — success
• New vitest cases: 82 new tests across 7 layers
  • v1 regex bridge & UI integration
  • Part A: fzy scoring (prefix bonus, consecutivity, gap penalty)
  • Part B: character position tracking (matchedIndices)
  • Boundary & edge cases (empty docs, Unicode, regex metacharacters)
  • Cross-API consistency (Scored vs Detailed alignment)
  • Performance benchmarks (50K doc <500ms)
  • Algorithmic invariants (monotonicity, determinism, subset properties)
• Manual UI: Fuzzy checkbox renders and highlights matches in electron-demo

Scoring Algorithm Invariants Verified / 已验证计分算法的不变量

• Prefix dominance: prefix match scores > any non-prefix match
• Consecutive > gapped: identical chars with contiguity outscore same chars with gaps
• Length monotonicity: shorter spans score ≥ longer spans at same positions
• Determinism: same input → identical output across calls
• Case-sensitive subset: CS_results ⊆ CI_results

---

Compliance / 合规自检

• CLA signed ✅ — email verified on GitHub
• AI disclosure: This PR was developed with AI assistance (CodeBuddy) used for code generation, test authoring, and documentation. Algorithm design, architecture decisions, and final review were performed by the human contributor.
• New dependencies: none
• No build artifacts committed
• No secrets

Checklist / 检查清单

• Title follows Conventional Commits
• Public API changes update package README / types
• Touched live-preview-table.ts — N/A
• New capability / breaking change → OpenSpec proposal linked — N/A
• Change aligns with project scope (GOVERNANCE.md §4)

Screenshots / Recordings · 截图或录屏 (UI changes)

• feat_search-fuzzy_UI_change_1.png — Electron Demo 中 Fuzzy checkbox 及高亮效果

• feat_search-fuzzy_API_testing.png — API 验证脚本控制台输出 (4/4 通过)