keeper: add reserve/commit/mark journal primitives

[mfw78]

What

Add the durable journal primitives to nexum-sdk's Journal: reserve, park, commit, release, mark, reserved, plus the Mark and Reservation types. Marker values are tag-first so a reservation and a commit can never collide by presence: RESERVED = [0x01] ++ be64(next_eligible_epoch_s) ++ body, COMMITTED = [0x02], and a legacy empty marker reads as committed for back-compat.

mark classifies by value: absent is None, empty or a leading 0x02 is Committed, and any other first byte (including a corrupt tag) is Reserved, so an unreadable marker forces a later reconcile rather than a skip. reserved enumerates exactly that Reserved set with each body and backoff.

This PR is dormant: it adds the primitives and retains record/contains for the observe path, and changes no sweep. The sweeps adopt these in the follow-up that also adds the reconcile pass, so a reserve write is never shipped without the scan that resolves it.

Why

The submit-dedup journal needs to distinguish an in-flight reservation from a settled receipt to close the post-submit-fault duplicate window without introducing a silent drop. contains cannot, since presence alone conflates the two. mark reads the value; its rustdoc now warns that contains must not guard a reserve/commit journal.

Testing

cargo fmt --all -- --check, cargo clippy --workspace --all-targets --all-features --locked -D warnings, all 17 guest wasms, cargo nextest run --workspace --all-features --locked (the new L1 unit tests: mark distinguishes reserved/committed/absent/legacy-empty/corrupt-tag; the mark/reserved agreement invariant; reserve/commit/release/park round-trips), cargo test --doc, RUSTDOCFLAGS=-D warnings cargo doc --workspace --no-deps --locked, scripts/check-venue-agnostic.sh, scripts/check-cow-orderbook-only.sh.

AI Assistance

Implemented with Claude Code.

Closes #538