fix(tooling): restore Fallow function coverage

[DavidBabinec]

What changed

• derive Istanbul fnMap/f entries from the TypeScript source AST when Bun LCOV omits per-function FN/FNDA records
• preserve native LCOV function records when another producer supplies them
• retain Bun DA statement/line coverage and emit full source ranges for inferred functions
• make the converter import-safe and add focused regression tests for native records, AST fallback, hit counts, ranges, and statement coverage

Why

Bun writes FNF/FNH aggregate totals and DA line hits, but no individual function records. The previous converter therefore generated 0 functions, so Fallow matched 0 of 28,000+ analyzed functions and fell back to estimated coverage for every CRAP score.

Impact

A fresh run now emits 12,966 Istanbul functions across 1,524 covered files. Fallow 3.9.1 directly matches 9,619 analyzed source functions instead of 0; files absent from Bun coverage continue to use Fallow's normal estimate.

Verification

• bun test scripts/lcov-to-istanbul.test.ts (3 pass)
• bun run test:coverage (6,257 pass; converter emitted 1,524 files / 12,966 functions)
• npx fallow health --coverage .coverage/coverage-final.json --format json --quiet --top 1 (coverage_model: istanbul, istanbul_matched: 9619)
• bun run lint
• bun run build