feat(flow): persist versioned plans, checkpoints, and recovery decisions

[szmyty]

Flow previously kept execution history in memory, so a host restart lost the distinction between prepared, interrupted, and accepted work. This adds a durable coordinator for prepared process steps and the state foundation required by #31.

• Add eight closed flow.run-*/v1 contracts for plans, state, checkpoints, artifacts, authority, validation, recovery, and snapshots; advance the provisional contract set to 0.7.0.
• Persist intent and authority before launch, then persist success only after the existing transcript and artifact acceptance gates pass. Keep the in-memory seams available.
• Commit immutable, digest-linked snapshots under an OS-backed workspace lock. Reopen validates the complete retained history and refuses partial writes, corruption, unsupported schemas, and concurrent opens.
• Bind eligibility to the complete plan, input bytes, provider/version, capability, actual configuration, authority, validator identity, and freshly observed artifacts. Recovery requires a recorded, explicit choice; completed steps cannot be launched again.
• Document privacy, retention, migration, rollback, and filesystem guarantees in the canonical architecture, integration guide, and proposed ADR-0011.

Validation (current review head 948eb4dfeb17e701bae61978bd6d593d28ee85e8):

• Rust 1.85.0 and stable: strict Clippy and all-target tests passed, including all 81 acceptance scenarios twice in fresh roots.
• Commit interruption, abrupt exit/reopen, workspace contention, cancellation/timeout, stale evidence, failed terminal commits, explicit retry/abandon, record budgets, and privacy tests passed.
• All 24 contracts, 34 positive fixtures, 21 negative fixtures, Python regression tests, deterministic sources, and repository validators passed.
• Documentation tests, three executable examples, and package verification passed locally (--allow-dirty before publishing the identical tree).
• Hosted CI run 36211631846 passed at head 948eb4dfeb17e701bae61978bd6d593d28ee85e8: Rust 1.85.0, Rust stable, repository validators, macOS durable state, and Windows durable state. Both Linux jobs also passed documentation, examples, and clean package verification.
• The first Windows run exposed a test helper reading the exclusively locked coordination file; it now checks that empty file's metadata. The portable suite and strict Clippy passed again locally. Platform jobs select stable Rust explicitly; production behavior is unchanged.

Limits: this is a trusted local, single-writer store. Windows guarantees process-crash consistency, not power-loss durability. Digests are correlation evidence, not authentication. V1 never repairs/prunes state or repeats uncertain effects automatically. General scheduling, downstream invalidation, real adapters, and the public CLI remain later checkpoints. ADR-0011 is proposed for maintainer review.

Rollback: revert this implementation while preserving run workspaces and artifacts. Older state or code never authorizes repeated provider effects; resolve uncertain work before restoring a prior workspace.

Closes #49. Parent #3; suite roadmap #11. After merge and green default-branch CI, #31 is next.