Unit artifact cache schema

Layout

Unit artifacts live under {project}/obj/beskid/cache/salsa/:

Path	Content
manifest.json	grammar_rev, compiler_version, schema_version, persisted_units
units/{content_fp}/meta.json	UnitArtifactMeta (logical name, source path, source length, imports)
units/{content_fp}/ast.bin	Postcard Spanned<Program> snapshot
units/{content_fp}/hir.bin	HIR cache marker (HIR rebuilt from AST on cold load)

Legacy obj/beskid/cache/units/*.json records are retired; tooling ignores them.

Fingerprint rule

content_fp = hash(grammar_rev, source_bytes) — not absolute path. Path is metadata only. grammar_rev is the workspace constant shared by beskid_pipeline, beskid_artifacts, and beskid_queries.

Invalidation

1. File edit — FileText revision bump clears fingerprints for old and new content; reverse_dependents propagates to import closure.
2. Grammar bump — GrammarRevision input invalidates all unit tracked queries globally.
3. Lockfile / manifest — ProjectSession.lockfile_digest change invalidates program_assembly for that entry.

Hot path

Production assembly uses Salsa in-memory unit_cache (parse + HIR tokens). Disk artifacts are read on cold start; writes occur on direct-parse paths without the Salsa delegate. Manifest persisted_units is refreshed once per assembly batch, not per unit.