Replace the hardcoded API-Error check with a target-keyed BackendErrorPatternLookup
plus a BackendErrorSink, mirroring the existing ExhaustedPatternLookup/ExhaustionSink
pair. Classifies in all three paths (normal block, #211 raw-scrape fallback, and the
fleetd#164 MIN_TURN_NANOS floor). The sink fires only after Rendezvous.resolveFailure
wins for the exact waiter. A target with no configured pattern still falls back to the
narrow (?i)\bAPI Error\s*: compatibility pattern. Existing constructors keep compiling
via BackendErrorPatternLookup.legacy() / BackendErrorSink.none() defaults.
Public send result is unchanged (still a failed send) — the typed sink event is the
internal seam Unit 5 will consume.
Dai Ha
776743cbe2