feat(settlement): emit a batch-summary event from settle_all

[thlpkee20-wq]

Closes #302

`settle()` and `settle_all()` already emit a
`settled(agent, service_id, requests, billed)` event per settlement —
`settle_all` publishes one per service in the sweep. This adds a single
`settl_all(agent, count, total_billed)` event after the loop, so
indexers can track a full `settle_all` drain without summing every
per-service `settled` event themselves.

• `count` is the number of services swept (including zero-billed ones,
  matching `settle_all`'s existing "drain and stamp even when usage is
  zero" contract).
• `total_billed` saturates at `i128::MAX`, like every other lifetime
  counter in this contract.
• No topic collision: `settl_all` (9 chars, within the `symbol_short!`
  limit) doesn't match any existing topic.
• No change to fund movement — this only adds a read-side event.

Test plan

• `cargo fmt --all -- --check`
• `cargo clippy --all-targets -- -D warnings`
• `cargo test` — 337 passed, 1 failed. The one failure is the same
  pre-existing, unrelated test covered by fix(test): correct cross-call event-count assumption in ownership-transfer test #315.

[mikewheeleer]

looks good @thlpkee20-wq — well scoped and easy to review. merging 👍