Add durable external-action settlement protocol - #704

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@crates/warp-core/src/causal_wal.rs`:
- Around line 9626-9640: Centralize external-action transaction-kind to
record-kind mapping in a shared WalTransactionKind accessor, such as
external_action_record_kind() -> Option<WalRecordKind>. In
crates/warp-core/src/causal_wal.rs:9626-9640, update
validate_transaction_semantics to use the accessor while preserving the
exactly-one-frame validation. In
crates/warp-core/src/external_action.rs:1446-1470, replace
external_action_frame’s local expected match with the same accessor so both
validation paths share one lifecycle mapping.

In `@crates/warp-core/src/external_action.rs`:
- Around line 772-794: The Merkle mutation path currently traverses the same
256-level path twice, in root_digest_with_entry and refresh_merkle_path.
Collapse these operations into one traversal that computes each parent hash,
applies the corresponding node writes, and returns the resulting root; update
callers to use that combined result while preserving existing depth, sibling,
and empty-hash behavior.
- Around line 1432-1444: Change record_external_action_request,
claim_external_action, and admit_external_action_settlement to accept and mutate
a caller-owned RecoveredExternalActionIndexV1, advancing it only after each
successful commit instead of invoking full WAL replay. Retain
recover_external_action_index_from_store and its recover_from_frames_and_commits
path exclusively for cold recovery and initial index construction, avoiding
repeated payload decoding, allocations, and Merkle rebuilding on the kernel
request path.

In `@crates/warp-core/tests/external_action_protocol_tests.rs`:
- Around line 310-323: Replace the broad is_err() assertions in
external_action_protocol_tests.rs at lines 310-323, 338-351, 375-385, and
411-411 with exact Err(...) equality checks. At 310-323 assert the
request/authorization binding error; at 338-351 assert the
missing-lease-evidence variant; at 375-385 assert
ExternalActionProtocolErrorV1::MissingExternalEvidence; and at 411-411 assert
the frontier-mismatch variant, preserving each test’s existing setup.
- Around line 527-548: The current test only verifies invalid attempt ordinal
handling, not prevention of multiple claims for one request. Extend the relevant
external-action protocol test to record a request, perform an ordinal-0 claim
successfully, then submit a second ordinal-0 claim using a fresh lease digest;
assert the specific single-claim obstruction error and verify the store commit
count remains unchanged after the rejected claim.
- Around line 899-966: Update CommitFailingStore::flush_commit to determine the
current commit count without cloning the full commit vector. Use an existing
WalStorePort or InMemoryWalStore count/length accessor if available, preserving
the fail_on_commit_ordinal comparison and normal commit behavior.