Incremental rebuild loses complexity/CFG/dataflow on native engine

[carlos-alm]

Bug

During 1-file incremental rebuilds with --engine native, complexity metrics, CFG blocks, and dataflow edges for the changed file are purged but never re-computed. The data silently disappears until the next full build.

Evidence

BUILD-BENCHMARKS.md v3.1.4 1-file rebuild phases:

Phase	Native (1-file)	WASM (1-file)	Native (full)	WASM (full)
Complexity	0.1 ms	0.1 ms	15.6 ms	76.5 ms
CFG	0.1 ms	0.1 ms	126.2 ms	124.5 ms
Dataflow	0.2 ms	0.2 ms	99.6 ms	98.4 ms

0.1ms is the cost of importing the module and iterating zero files — no actual analysis runs.

Root cause

1. purgeFilesFromGraph deletes the changed file's rows from function_complexity, cfg_blocks, cfg_edges, and dataflow tables
2. The native parser produces symbols without a _tree (WASM tree-sitter tree)
3. The unified walk in ast-analysis/engine.js:108-109 skips files with no _tree:

   if (!symbols._tree) continue; // No WASM tree — native path handles it

4. The buildXxx functions then check for pre-computed data on definitions (d.complexity, d.cfg?.blocks), find none (native doesn't compute these), and also skip
5. ensureWasmTrees (line 79-98) only triggers for dataflow and only when !symbols.dataflow — but native provides basic dataflow data, so it's skipped too

Result: purged data is never replaced.

Affected scope

• Only affects incremental rebuilds (1-file or scoped), not full builds
• Only affects native engine (--engine native or --engine auto on systems with the native addon)
• WASM engine is unaffected because it always has _tree available

Proposed fix

In ast-analysis/engine.js, call ensureWasmTrees for files that lack a _tree when any of complexity/CFG/dataflow are enabled — not just for dataflow. The existing gate:

if (doDataflow) {
  // only checks !symbols.dataflow
}

Should be broadened to:

if (doComplexity || doCfg || doDataflow) {
  // check for any file missing _tree that needs analysis
}

This ensures the WASM tree is available for the unified visitor walk even when the native engine handled parsing.