feat: Rust core engine via napi-rs (Phase 1)

[carlos-alm]

Summary

Add a native Rust parsing engine via napi-rs as an optional high-performance alternative to the existing WASM pipeline. The WASM path remains the always-available baseline; the native engine activates automatically when platform binaries are installed.

Phase 1 — Rust core (napi-rs dual-engine)

• All 11 language extractors (JS/TS/TSX, Python, Go, Rust, Java, C#, Ruby, PHP, HCL) ported to Rust with a SymbolExtractor trait
• Rayon-based parallel file parsing
• 6-level import resolution with confidence scoring
• Tarjan's SCC cycle detection
• Incremental parse tree cache for watch mode

Phase 1.2 — Unified API

• parseFileAuto, parseFilesAuto, getActiveEngine as the single entry point in parser.js
• Builder and watcher use the unified API — no direct native imports

Phase 1.3 — Incremental parsing

• Native ParseTreeCache exposed via napi-rs for watch mode old-tree hints

Phase 1.4 — Native import resolution

• resolveImportPath and computeConfidence extracted to src/resolve.js with native dispatch
• Batch pre-resolution of all imports in a single native call

Phase 1.5 — Graceful degradation & diagnostics

• --engine flag plumbed to watch command
• codegraph info diagnostic command
• CI workflows for cross-platform test matrix and native build pipeline

Changes

• 40 files changed, ~5,500 lines added
• New Rust crate at crates/codegraph-core/
• New JS modules: src/native.js, src/resolve.js
• Refactored: src/builder.js, src/parser.js, src/cli.js, src/watcher.js, src/cycles.js
• CI: build-native.yml (4-platform matrix), ci.yml
• Tests: engine parity, unified API, import resolution parity, build parity, incremental cache

Test plan

• npm test — full WASM test suite passes (no native required)
• Cross-engine parity tests pass when native binary is available
• Build parity test confirms identical graph output from both engines
• codegraph info reports correct engine status
• --engine wasm forces WASM even when native is available
• Watch mode falls back gracefully when native unavailable

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