feat(server): file a papercut when the stall watchdog flags a turn

[nohat]

Part of #22. Wave 2, after #26 for useful content.

Problem

A papercut exists only if I notice a problem and tap. The stall that hurt most (#1) is already detected by the server, and the evidence is freshest at that moment. Waiting for me costs minutes of trace rotation, and a hung client cannot report at all.

What the code shows

• ThreadTurnWatchdog.ts flags a running turn silent for 10 minutes (STALLED_TURN_THRESHOLD_MS), appends provider.turn.stalled, and logs orchestration.turn.stalled (around lines 84-132). It covers claudeAgent only (WATCHED_PROVIDERS) and skips turns waiting on approvals, user input, or background work.
• docs/fork/papercuts.md still says the server has no stalled-turn detection; stale, corrected in the decision commit for feat(papercuts): a local feedback loop from one-tap report to verified fix #22.

Plan

1. When markStalled succeeds, create a server-originated papercut: no screenshot, no client evidence, a server value added to PapercutClientSurface, a one-line summary ("stalled turn, idle N s"), and the full server snapshot from feat(papercuts): capture the server's full picture at report time #26.
2. At most one per thread and turn (deterministic report id from thread, turn, and stall time). Resume then stall again on a later turn creates another.
3. Failure to create never affects the sweep (catchCause, as the existing append).
4. No notification. The triage digest (feat(papercuts): a triage job that turns new reports into issues #30) is how I hear about it.

Acceptance

• Test with the existing sweep harness (ThreadTurnWatchdog.test.ts): a stalled thread yields exactly one record across repeated sweeps; a new turn yields another; a watched-out provider yields none.
• The record's overlay can be marked and triaged like any other.

Surfaces

Server only. Contract: one enum value. Providers: inherits the watchdog's Claude-only scope; widening it is its own decision. Other server-side triggers (provider start failure, a send that never settles) need an exhibit before they are added.

Related

#22, #26, #1, #30.