docs(sqs/throttle): refresh stale comments on idle-evict constants

Round-3 Claude review on PR #679 caught two stale comments left over
from the runSweepLoop refactor:

throttleIdleEvictAfter said "no goroutine; lookups call sweep()
opportunistically" -- both clauses are now false (runSweepLoop is
the goroutine, hot-path charge() never calls sweep()).

throttleEvictSweepEvery said "bounds how often the sweep runs from
the hot path" -- the hot path no longer runs sweep at all; this
constant is now just the background ticker interval.

Both updated to describe the post-runSweepLoop design and to
reinforce that the sweep cost is amortised across the goroutine
ticker rather than concentrated on whichever request was unlucky
enough to trigger it (which was the old behaviour Gemini high
flagged in PR #679 round 1).

Author: bootjp

adapter/sqs_throttle.go

@@ -73,15 +73,18 @@ const throttleHardCeilingPerSecond = 100_000.0
 const throttleMinBatchCapacity = float64(sqsBatchMaxEntries)
 
 // throttleIdleEvictAfter is the idle window after which a quiet bucket
-// is dropped from the in-memory store. The evictor fires lazily — there
-// is no goroutine; lookups call sweep() opportunistically. A queue that
-// resumes activity rebuilds its bucket from the meta record at full
-// capacity, matching the failover semantics documented in §3.1.
+// is dropped from the in-memory store. A background goroutine
+// (runSweepLoop) fires the eviction sweep on each
+// throttleEvictSweepEvery tick; the hot path never calls sweep().
+// A queue that resumes activity rebuilds its bucket from the meta
+// record at full capacity, matching the failover semantics
+// documented in §3.1.
 const throttleIdleEvictAfter = time.Hour
 
-// throttleEvictSweepEvery bounds how often the sweep runs from the hot
-// path so a flood of misses on a many-queue cluster cannot turn the
-// idle-evict cost into a per-request hot path.
+// throttleEvictSweepEvery is the interval at which runSweepLoop fires
+// the idle-evict sweep in its background goroutine. The hot-path
+// charge() never calls into the sweep so a many-queue cluster pays
+// the O(N) cost only on the goroutine's tick, never on a request.
 const throttleEvictSweepEvery = time.Minute
 
 // bucketKey is the in-memory map key.