cow: rename twap-monitor to ccow-monitor and enable mainnet

[mfw78]

Final car of the convergence train. Supersedes the rename outcome of #522.

With zero TWAP logic left in the module, git mv modules/twap-monitor modules/ccow-monitor as a provably mechanical PR, then enable mainnet. This is the tracker anchor for the generic monitor that #568 already treats as committed.

Why

Rename, not new-module-plus-delete: the 1200-line keeper suite and 995-line run suite travel through every commit, so there is no coverage gap and no dual-maintenance window. TWAP survives as watch scope and vectors, not code, exactly as #138 recorded. Mainnet enablement rides here because the manifest, engine config, and ops scripts all change names in the same sweep.

Scope

• git mv, crate name, module label "ccow".
• module.mainnet.toml (chain 1, three event rows); engine [chains.1]; wire the existing mainnet cow-venue adapter manifest.
• Sweep scripts/lib.sh:18, e2e-onchain.sh, baseline_latency.py:88, and the engine.*.toml rows.
• CI rg gate: zero surviving 0xfdaF… or COMPOSABLE_COW references.

Done when

• The rename commit is mechanical (rename-detected, no logic diff).
• The mainnet manifest is live and the module subscribes to the three fork events on chain 1.
• The CI grep gate is green.
• Both test suites pass unmodified apart from path and name changes.

Blocked by: #655.

[mfw78]

Watch-tower harvest delta: first mainnet enablement runs open-admission against a permissionless registry, because the Tier-3 queue and interim digest allowlist are filed by #659, not built. The #653 gas cap, OOG-park, and response-size caps are then the only line of defence.

Additional done-when:

• The enabling PR body records an explicit go/no-go on mainnet without the interim allowlist, as an accepted-risk decision (or gates enablement on the allowlist landing first).

Mainnet enablement is additionally blocked by the resume-cursor car: the mainnet manifest variant must declare resume on both chain-log rows, matching the base module, or downtime silently loses mainnet watches. Blocked by: #660.