feat: generalize framework artifact pipeline foundations

[forhappy]

Summary

Introduce the framework-neutral Trail artifact pipeline described by the generalize-framework-artifact-pipeline OpenSpec change.

This PR now completes 31 of 87 planned deliverables. It remains a foundation PR, not a claim that the full v2/v3 resolver, execution, export, and native-backend rollout is complete.

What changed

• return typed ready/resolvable/blocked/unsupported/ambiguous environment proposals, including manifest-only Cargo and Node projects
• add bounded resolution plan and immutable resolution snapshot contracts
• add durable resolver attempts with exact process/generation fences, heartbeat, cancellation, singleflight, bounded output capture, credential redaction, failure receipts, and dead-owner recovery
• revalidate pinned source inputs and executable identity at attempt start and successful completion
• add desired-key v2 identity material and deterministic invalidation-edge explanations
• add content-addressed directory, file, blob, FastCDC chunk, chunk-list, tree-root, and envelope objects
• seal legacy workspace layers into verified CAS shadows while keeping v1 identity explicit
• reconstruct missing layer materializations from authoritative CAS content during workspace-open recovery
• add crash boundaries across reservation, object publication, envelope readiness, materialization, and activation
• quarantine divergent tree roots for one desired key/trust scope, retain evidence and holds, and support explicit private resolution
• extend fresh schema-v1 creation with artifact snapshots, objects, envelopes, attempts, attestations, quarantines, holds, generation bindings, and resolver failure receipts
• invalidate generation bindings and layer shadows in foreign-key-safe order during backup restore while preserving authoritative CAS content
• make layer GC remove the legacy layer/CAS shadow mapping atomically while leaving authoritative content for future reachability GC
• add OpenAPI and CLI/HTTP/MCP discovery parity coverage

Why

Trail's existing environment layers couple correctness identity to one key-derived physical cache directory. That prevents framework-neutral resolution, deduplication of equal content across different desired keys, safe detection of non-reproducible producers, and reliable reconstruction of evicted materializations.

The new model separates desired identity, pure tree content, artifact envelopes, and future generation bindings. Framework names do not grant reuse; adapters must provide bounded evidence and unproven portability falls back to lane-private, non-reusable output.

User and developer impact

• manifest-only Cargo and Node repositories are reported as blocked proposals with recovery guidance instead of being silently omitted
• resolver lifecycle state now survives process death and workspace reopen without retaining credential values
• newly published legacy layers have verified CAS authority and can be reconstructed after materialized cache loss
• backup restore preserves authoritative artifact content while dropping rebuildable generation/layer projections
• divergent producers fail closed rather than silently selecting one tree
• existing WorkspaceLayerKeyV1 identities remain explicitly legacy and are never presented as desired-key v2

The remaining 56 OpenSpec tasks include actual resolve-all/component execution, repository schema v2, adapter protocol v3, native lazy materialization, CAS reachability GC/accounting, attestations/capability enforcement, public artifact CLI/HTTP/MCP operations, framework migrations, documentation, and real-repository/native qualification.

Validation

• openspec validate generalize-framework-artifact-pipeline --strict
• cargo fmt --all -- --check
• cargo check --workspace --locked
• cargo clippy --workspace --all-targets --all-features --locked -- -D warnings
• library suite: 819 passed, 0 failed, 4 intentionally ignored
• main binary suite: 61 passed, 0 failed, 1 intentionally ignored
• E2E suite: 223 passed, 0 failed
• schema-v1 hard-cutover: 18 passed
• changed-path producer inventory: 24 passed
• changed-path activation: 9 passed
• lane initialization fault suite rerun: 28 passed
• resolver lifecycle tests pass with the default scheduler and RUST_TEST_THREADS=1
• artifact CAS benchmark passed; a 16 MiB successor reused 12 of 13 chunks and added about 690 KiB of unique object bytes on the qualification host

One aggregate cargo test --workspace --locked run encountered a nondeterministic macOS native-observer failure in the 64-observer initialization stress test after all affected artifact, restore, schema, and E2E cases had passed. The exact test passed in isolation, and the complete 28-test initialization-fault binary passed on rerun. CI native-host gates remain authoritative.

All compiling commands used /Volumes/Workspace/crabbuild-target/trail-optimize-large-repo-lanes as the checkout-specific CARGO_TARGET_DIR.