feat(slasher): slash checkpoint equivocation between P2P and L1 (A-980)

[PhilWindle]

Summary

• The archiver already detects when a locally-stored proposed checkpoint disagrees with the L1-confirmed checkpoint at the same slot (in tryBuildPublishedCheckpointFromProposed), but until now it only logged the divergence and evicted the local entry. Both sides are signed by the slot proposer, so the proposer equivocated.
• Emit a new `CheckpointEquivocationDetected` event from the archiver when this divergence is detected.
• Add `CheckpointEquivocationWatcher` in the slasher that subscribes to the event, resolves the slot proposer via the epoch cache, and emits a `DUPLICATE_PROPOSAL` want-to-slash event.
• This closes the gap where a proposer broadcasts one checkpoint via gossip and submits a different one to L1 — the existing `AttestationPool` P2P duplicate detection can't see this because only one of the two proposals is gossiped.

Fixes A-980.

Test plan

• New unit tests in `yarn-project/slasher/src/watchers/checkpoint_equivocation_watcher.test.ts` cover: emit on divergence, no-emit on missing proposer / zero penalty / after stop, dedup of repeat events, separate offenses per slot.
• The end-to-end scenario already exists in `yarn-project/end-to-end/src/e2e_epochs/epochs_equivocation.test.ts`. Its TODO around asserting the slash is still gated on enabling the slasher in the harness (separate plumbing) — comment updated to reflect that detection is now implemented.