Canonical intermediate-state FSM: enum, legal transitions, deterministic reducer

[SollanSystems]

Today the active state in state.json is an arbitrary string and transitions are prose. Kernel must own legal transitions ("agents propose; the kernel disposes" — ADR 0001).

Scope:

• Canonical intermediate-state enum + legal-transition table in the kernel.
• Validation in contract.py (doctor) + enforcement in emit writers.
• ISO-8601 UTC timestamps stamped by state/terminal writers (roadmap: normalize IDs and timestamps).
• Deterministic reducer lands with the EventStore issue; this issue covers the FSM definition + validator.

[SollanSystems]

Delivered across two merged PRs (this issue's own scope note anticipated the split):

• FSM enum + legal-transition table + doctor validation + emit enforcement + ISO-8601 UTC writer timestamps — PR feat(kernel): canonical intermediate-state FSM, unknown-state validation, writer timestamps #59 (squash e5c441c): loop/fsm.py canonical intermediate states + is_legal_transition, unknown-state validation in contract.py, writer timestamps, terminate() now stamps state:"terminal".
• Deterministic reducer — PR feat(kernel): loop-engineer/event@1 EventStore + SQLite/WAL + deterministic reducer (#50) #62 (squash 3952641): loop/reducer.py folds event@1 streams enforcing FSM transition legality via the same loop.fsm functions the writers use, plus G1 all-required completion and terminal immutability at replay.

Suites at 3952641: extras 618 passed/15 skipped; pyyaml-only 588/45.