Bound the drain's send, and stage what times out

[scgopi]

Closes the send-path half of the stable gate's NO-GO on 0.1.64: the delivery chain in drainPendingFollowUps now has the same deadline the presence read got in #316, and whatever the deadline drops is staged to memory rather than stranded or silently lost.

Defect (fails without the change)

drainPendingFollowUps awaits deliverToSession unbounded in its walk — ZmxSessionLauncher.send → PTYProcessSession.waitCollectingOutput, three sequential zmx send beats per message — so one loop's hung send held the queue for the drain lease's entire 300s while every other loop's staged mail sat unmoving, with nothing logged. The stable-release check's DeliveryWedgeTests reproduce it against 0.1.64-beta6: control delivers in the window, one hung send freezes the bystander.

Red (82b86a6's tests, drain hunk reverted, private DerivedData): exit 65 — aHungDeliveryNoLongerFreezesTheBystandersFollowUps (bystander never delivered) and aTimedOutFollowUpIsStagedToMemoryNotSilentlyDropped (memory never written) both fail; the control passthrough and the three read-side inverted tests (read bounded at 45s, timed-out item still owed, poll reaches loops behind a hung one) pass.

Green: same tests exit 0 after 41315a9.

Fix

• The drain's single deliverToSession await now runs under withDeadline(deliveryDeadline) — the same withDeadline Bound the presence read, and let the drain alone decide delivery (#311, #304) #316 introduced, loser abandoned rather than cancelled-and-waited (cancel ignores a task parked on a pipe that will never close).
• Deadline passed/staged-not-requeued: staged(_:) writes the canonical "follow-up staged:" memory entry exactly once and the item leaves the queue — the Mailroom mirror already carries the content, so the target reads it at its next wake instead of retrying a send that may only ever hang.
• Same treatment for a delivery that returns false (the transport failed outright): the old drain ignored the result and silently dropped the item without any memory record.
• delivery-stall logged via #289 diagnostics when a deadline passes — frozen no longer looks identical to working.
• The lease stays as the backstop for awaits outside the drain's walk (stop requests, nudges). Default span 45s, same as presenceReadDeadline — a bystander behind one wedged target waits one span, not 300s and not the process's life.

Gate on 41315a9 (private DerivedData)

• xcodebuild test exit 0 — 1692 tests / 180 suites / 0 failures (beta6 baseline 1686; +6 from the new DrainWedgeVerificationTests file carrying the read-side inverted tests and the delivery suite)
• graphcode-cli build exit 0; graphcoded build exit 0
• swiftlint 0 errors; swift-format clean (in-place format pass applied)
• scripts/cli-smoke.sh exit 0 (five verbs; run against a fresh swift build in an isolated scratch — the shared .build's debug CLI was stale, which was a shuttle from before, not a gate failure)
• Linux: to be read from this PR's checks when they land — noted here so it is checked, not inferred.