feat(aztec-node): force sentinel for validators + offense collection

[spalladino]

Motivation

The sentinel is required for the network to reach consensus on slashing offenses — it's a core duty of participating in consensus, not an optional add-on. Yet SENTINEL_ENABLED defaulted to false, so a validator could run without one and silently degrade the network's ability to enforce its own rules.

While fixing that, two adjacent issues surfaced:

• Whether the node runs the sentinel and the slashing-detection watchers was gated on a prover-only flag (enableProverNode && disableValidator). That coupling is wrong: those subsystems should depend on validator status, not on whether the prover is enabled.
• A non-validator (e.g. an RPC or full node) had no way to collect and inspect offenses, because the only component that persists them lives inside the slasher client, which was created only for validators.

Approach

• Force the sentinel for validators. createSentinel now self-gates: if the node runs a validator it always creates the sentinel (ignoring SENTINEL_ENABLED); otherwise it respects SENTINEL_ENABLED (default false). It logs which case applies so operators understand why a SENTINEL_ENABLED=false is being overridden.
• Decouple subsystem gating from the prover. Removed the proverOnly guard. The slashing-detection watchers and the slasher are now gated on collectOffenses = !disableValidator || enableOffenseCollection.
• Opt-in offense collection for non-validators. A new OFFENSE_COLLECTION_ENABLED flag lets a non-validator run the watchers plus a read-only slasher client (it never writes to L1 — voting/execution flow through the sequencer publisher, which a non-validator lacks) to collect and serve offenses over the existing getSlashOffenses admin RPC. The spartan deploy enables it by default, mirroring SENTINEL_ENABLED.
• Make the attested-invalid-proposal watcher work without a validator. Lifted the invalid-proposal / equivocation slot tracking out of ValidatorClient into ProposalHandler, which now implements InvalidProposalSlotSource. The handler populates this from its all-nodes proposal handlers, so any node that re-executes proposals (the default) can feed the watcher.

API changes

• New node config enableOffenseCollection (env OFFENSE_COLLECTION_ENABLED, default false). Named distinctly from the existing L1-deploy AZTEC_SLASHER_ENABLED to avoid confusion.
• Validator nodes now always run the sentinel regardless of SENTINEL_ENABLED.

Changes

• aztec-node: createSentinel forces the sentinel for validators and logs the reason; server.ts removes proverOnly, gates watchers + a split-out read-only slasher on collectOffenses, and wires the attested-invalid-proposal watcher to the proposal handler.
• aztec-node (config) / foundation: new enableOffenseCollection / OFFENSE_COLLECTION_ENABLED flag and env-var entry.
• validator-client: ProposalHandler now owns the invalid-proposal / equivocation slot tracking and implements InvalidProposalSlotSource; ValidatorClient delegates to it, preserving validator behavior.
• spartan: OFFENSE_COLLECTION_ENABLED wired through the chart value, pod template, network defaults, terraform variable, and deploy script, defaulting on (mirrors SENTINEL_ENABLED).

Fixes A-1242