roadmap: Chapel L2.3 cancel-token preemption — gated on L1 Cap'n Proto + cross-repo Chapel CT SDK

[hyperpolymath]

Origin

CLAUDE.md ("Project-specific context" — Next v2.2+ section):

L2.3 (cancel-token through tryProver for mid-flight preemption) is the next sub-wave; L2.4 mutation parallelism / L2.5 multi-locale / L2.6 numeric hot paths / L2.7 bench remain hard-gated on L1 Cap'n Proto and (for L2.5) a cluster runtime — see docs/handover/TODO.md.

And the endpoint-target row for Chapel:

| "Chapel fully supported" | ... L2.3 cancel-token preemption next | dispatch.rs picks Chapel-parallel dispatch by config; runtime init + cancellation + error propagation wired; ≥1 OoM speedup on portfolio solves |

L2.3 is the next Chapel sub-wave after L2.2 (#133 rehabilitation arc, MERGED 2026-05-30).

Scope

Wire a cancel-token through tryProver in src/chapel/ so a first-success-wins parallel dispatch can preempt slower in-flight provers:

• Cancel-token type at the Chapel boundary (matched to the Rust side's cancellation primitive)
• Propagate cancellation through tryProver so an in-flight prover invocation can abort
• Error-propagation semantics: cancelled prover returns a Cancelled variant rather than Failure
• Property test in proofs/agda/ParallelSoundness.agda extended to cover the cancellation-safety invariant (already partially formalised per the L2.2 closure note)
• L2.3 specifically targets mid-flight preemption — the current parallelProofSearchSpeculative waits for the first success but does not stop the other in-flight workers

Estimated: small (~3-5 PRs) once unblocked.

Dependencies

• HARD-GATED on L1 Cap'n Proto (issue roadmap: Cap'n Proto IPC schema finalisation (Stage 5a — OWNER-GATED, needs Opus sign-off) #194 Stage 5a) — cancel-token must traverse the wire protocol cleanly
• HARD-GATED on cross-repo Chapel CT SDK — a Chapel-side cancel-token SDK does not yet exist in the broader Chapel ecosystem; this work either adopts an upstream SDK if one appears or stands up our own
• Stage 5 launch (roadmap: Stage 5 full launch (IPC + Chapel + Tier-4) — OWNER-GATED meta-tracker #195) — same wave coordination

Why filed

To make the L2.3 commitment trackable as a distinct work item; today it lives only in CLAUDE.md prose with no issue. The "Chapel fully supported" endpoint-target row cites L2.3 as the next sub-wave but has no tracker.