feat(index): record parser coverage receipts

[odvcencio]

Summary

• store per-file parser coverage receipts in index schema 0.3.0
• distinguish clean, partial, stopped, generated, and unknown parse outcomes
• expose coverage through canopy index coverage and the gts_coverage MCP tool
• propagate parser confidence into reports, changed-file reviews, and file guardrails
• attach scoped parser health to task-conditioned context bundles
• report bounded issue files and a unique untrusted-file count
• compare parser health between current and baseline executive reports
• rebuild stale caches when their schema predates coverage receipts
• surface coverage changes in watch baselines

The receipts preserve top-most ERROR and MISSING regions, parser stop reasons when available, recovered-region decisions, and generated-file fast paths. A clean receipt means no actionable parser gap was detected; it does not claim complete grammar or tag-query semantics.

Legacy context packing remains unchanged. Parser health is additive on task-conditioned bundle responses and bundle-mode JSON output.

Companion gotreesitter PR: #979. It exposes bound-tree analysis paths for a later dependency update.

Validation

• GOWORK=off go test ./...
• GOWORK=off go vet ./...
• GOWORK=off go test -race ./internal/indexcoverage ./internal/mcp ./cmd/canopy
• GOWORK=off go run ./cmd/canopy index coverage . --no-cache --limit 20
• live self-index: 302 files clean, 0 gaps, 0 parse errors
• live executive report: 302 clean files and 0 untrusted files
• live changed-only review: 24 indexed changed files, clean parser health, blast radius 31
• live context bundle: 77 selected items across 30 parser-clean files
• live current-versus-baseline parser-health comparison
• git diff --check origin/main...HEAD