Migrated from Method backlog

This issue was created from a legacy filesystem backlog card. GitHub Issues are now the live work tracker; repository docs remain Method evidence.

Source backlog: docs/method/backlog/v0.1.0/PLATFORM_contract-retention-and-semantic-lookup-seams.md
Original lane: v0.1.0
Original legend: PLATFORM

Original backlog card

Contract Retention And Semantic Lookup Seams

Status: implemented local boundary; retained evidence refs complete.

Depends on:

• Contract artifact retention in echo-cas
• Contract reading identity and bounded payloads
• 0018 - Contract-Hosted File History Substrate

Why now

Large retained artifacts and bounded readings need semantic lookup above raw
CAS identity. Full materialization is allowed as a cache, never as canonical
truth.

What it should look like

Keep echo-cas content-only, but add semantic contract refs above CAS:

• contract family;
• schema/type/layout identity;
• codec and hash algorithm;
• blob or fragment role;
• basis coordinate;
• retention tier.

Add a bounded blob reader seam if BlobStore::get Arc<[u8]> cannot support
bounded observers without full materialization. This is retained-payload lookup,
not a streaming subscription surface.

Core now also exposes retained evidence references above the byte-retention
layer:

• RetainedEvidenceCoordinate names contract identity, retained role, and
  semantic digest;
• RetainedEvidenceRef binds that coordinate to content hash and byte length;
• RetainedEvidencePosture returns available evidence or a typed
  MissingRetention obstruction.

Acceptance criteria

• Contract payloads can refer to retained blob fragments by CAS ref.
• Bounded contract queries can read retained byte ranges under a budget.
• Retained contract artifacts, receipt material, witness refs, reading
  envelopes, and reading payloads can be looked up by semantic coordinates.
• If retention is unavailable, lookup returns typed obstruction.
• GC or compaction cannot silently produce false successful retained readings.
• Cache hits are accepted only when the semantic coordinate matches.
• Retained reading refs are not query identities.
• Reading payload and reading-envelope references remain distinct roles.

Non-goals

• Do not redefine wormholes as rope chunks, portals, or state zoom.
• Do not make CAS content hashes stand in for semantic reading identity.
• Do not build a full distributed storage protocol in this slice.