fix: remove dead writes before panic in accept_intent

[Keengfk]

In accept_intent's expiry-check branch, two calls —
env.storage().persistent().set(...) and Self::bump_intent_ttl(...) — were made
immediately before panic_with_error!(&env, Error::IntentExpired). Because
Soroban discards all storage mutations from a panicking invocation, these
writes never committed. They were dead code that wasted CPU cycles staging a
write that would always be thrown away, and misled future maintainers into
believing the intent's state was being persisted before the panic.

What changed

intent_settlement/src/lib.rs

Removed the dead .set() and bump_intent_ttl() calls from accept_intent's expiry
branch. Replaced them with an inline comment explaining that no write is
possible at this call site — any mutation staged before a panic_with_error! is
discarded by the Soroban runtime — and that the durable Open → Expired
transition belongs exclusively to expire_intent.

intent_settlement/src/test.rs

Added accept_expired_intent_state_unchanged regression test. After a failed
accept_intent call on a past-deadline intent it asserts:

• The call returns Error::IntentExpired (existing behavior unchanged)
• get_intent still returns state: Open — no partial write committed
• intent.solver is still None — no partial write committed

This makes the expected observable behavior explicit and guards against the
dead-write pattern being reintroduced accidentally.

docs/111-expire-intent-event-coverage.md

• Status line updated to note the dead writes were cleaned up
• §2 updated to past tense — the dead writes have been removed, not just
  identified
• New §6 "Code Cleanup (dead write removal)" added, documenting what was
  removed and referencing the regression test

Testing

The existing accept_expired_intent_fails test is unaffected — the IntentExpired
panic behavior is unchanged. The new accept_expired_intent_state_unchanged
test adds the state-invariant assertion that was previously missing.

Out of scope

expire_intent itself is unchanged. It remains the sole function that durably
writes state: Expired to storage.

closes #279

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