chore(rivet): record the remainder of v1.5.0's scope before building it (#313)

[avrabe]

rivet release status v1.5.0 reported "✓ Cuttable" while the release contained exactly one artifact (Tier-2 slice 1). The gate was not wrong about that artifact — it was reading green because the scope was almost empty, and a release gate that passes on an empty scope is indistinguishable from one that passes on a finished release.

This is the same defect class the repository has now found five times in code — the trap gate with 19 passing tests and zero callers (#288), the premise hook with no producers (#323), the pin defended only by a comment (#318), a proof that had never run, and loom verify printing a checkmark without opening the file (#332). This is the first instance found at the requirements layer rather than in the code.

Change

Records the remaining #313 scope as six draft artifacts — Tier-2 slices 2–6 plus the exit conditions.

Before / after:

Release v1.5.0 — 1 artifact(s)      Release v1.5.0 — 7 artifact(s)
  verified 1                          draft 6, verified 1
✓ Cuttable                          ✗ NOT cuttable — 6 not yet verified

Three things pinned now because they are easy to get wrong later

• Slice 2 must keep div_s/rem_s asymmetric — div_s(INT_MIN,-1) traps, rem_s(INT_MIN,-1) is 0 — and must reject trap introduction as well as trap removal. A one-directional check accepts half the bug class.
• Slice 6 must prove the non-singleton valueSet path is reachable, not merely present, and must resolve Investigate: verify.rs opaque-float shared const name may unify distinct float ops (verifier soundness) #283 rather than transcribe it.
• The exit artifact is what makes the retirement checkable instead of declared, and it is not startable until both mode is clean.

Each artifact carries acceptance criteria written as assertions that must fail if the capability is removed, per #289.

No code changes. Scope only. rivet validate → PASS.

Refs #313, #283, #289, #300, #311, #332