Name the poll budget in a re-spawn note instead of the unbounded sentinel

[vjovanov]

Closes #118

What changed

The re-spawn note on a poll state printed the internal u64::MAX sentinel as
if it were the attempt budget:

Re-spawning Task plan.1 in state 'plan-approval': attempt 4 of 18446744073709551615; the previous attempt exited 75

It now names the bound that actually applies, poll.max_attempts:

Re-spawning Task plan.1 in state 'plan-approval': attempt 4 of 96 (poll.max_attempts); the previous attempt exited 75

The poll state's exemption from the visit attempt budget is unchanged — the
poll's own exhaustion rule (poll.max_attempts) is still the only thing that
ends that loop — and a non-poll state's re-spawn note is byte-identical to
before.

resolve_attempt_budget now returns a typed AttemptBudget (Visit(u64) /
Poll { max_attempts: u64 }) instead of encoding "exempt" as u64::MAX, and
respawn_note renders each form accordingly.

The spec sentence at
§FS-rhei-agents.3.2.1,
a pinning integration test
(run_poll_respawn_note_names_poll_max_attempts_not_the_sentinel), and the
changelog bullet are all in the same commit as the fix.