* fix(line): adopt durable ingress drain with ack gated on spool write
LINE acked webhooks 200 before processing events detached, so a crash or
dispatch failure silently lost inbound messages. Events now enqueue their raw
JSON into the channel ingress queue before the webhook responds; dispatch,
retry, dead-letter, and completion tombstones run through the core drain.
The in-memory replay guard is deleted; tombstone retention (30d/4096)
strictly covers its old 10min/4096 window, and message-id keys preserve the
guard keyspace so redeliveries under changed webhookEventIds still dedupe.
Autoreview: secret scanner false-positive on HMAC test fixtures
(test-channel-secret); full manual review of all five files + test matrix
performed instead. Part of the #109657 fleet adoption program (wave 1).
* style(line): oxfmt pass on drain adoption files
* fix(line): cap repeated ingress drain deliveries
* fix(line): preserve ingress lifecycle bounds
* style(line): satisfy ingress lint gates
Peter Steinberger