feat(artifacts): add lifecycle planning and provider seam

[jmagar]

Summary\n\n- add deterministic Artifact revision diffs and editable-workspace snapshots\n- add explicit, read-only update plans and a transport-neutral ArtifactProvider seam\n- add verified local provider acquisition with per-file/package byte budgets\n- add expected-base compare-and-swap head transitions and safe cross-platform atomic JSON replacement\n- preserve exact immutable revision reuse for repeated/reverted content\n- update Artifact spec, contract, implementation plan, and progress evidence\n\n## Safety / behavior\n\n- provider acquisition verifies path, size, SHA-256, component count, and local materialization budgets\n- update planning never mutates head, workspace, lineage, or stored bytes\n- stale head transitions fail with head_changed\n- snapshots reuse existing immutable revisions instead of rewriting historical metadata\n- no retired Stash/Marketplace/Fleet/Deploy product/protocol aliases introduced\n\n## Verification\n\n- hardened Artifact tests: 27/27 passed\n- full labby-runtime all-features tests passed\n- cargo clippy -p labby-runtime --all-features --all-targets --locked -- -D warnings\n- RUSTFLAGS=-D warnings cargo check -p labby-runtime --all-features --all-targets --locked\n- cargo deny check: advisories, bans, licenses, sources all passed\n- cargo fmt --all -- --check\n- git diff --check\n- labby docs check: 17 generated docs fresh\n- documentation link check: 347 local links verified\n- final detached post-doc verifier exited 0 with FINAL_OK\n\nPhase 1 landed in #462 at 8723910.

[jmagar]

Phase 2 current-main revalidation is complete on head 1fb73024c against base 52061b650.

• PR merge state: clean
• fresh check runs: 38 total, 0 active, 0 failures
• ci-gate: success
• full Linux Test: success
• Feature slice (gateway): success
• Feature slice (fs): success
• MCP focused regressions: success
• MCP 2026-07-28 conformance: success
• Code Mode runner smoke: success
• Generated docs: success
• Clippy: success
• MSRV: success
• Rustdoc: success
• Cargo Deny / Format / repository and fleet contracts: success

Local verification after rebasing onto current main also passed: full labby-runtime suite, Clippy -D warnings, RUSTFLAGS=-D warnings, cargo deny check, 17 generated docs fresh, and 347 local links verified.

No reviews or review threads are outstanding. No further branch mutation is needed for this Phase 2 slice.