feat: Add Non-polling Waiters (Deterministic Channels & Delta-Polling) #673

libtmux needs waits that distinguish cooperative completion, currently visible text and future observed output. Matching a prompt, echoed command or quiet interval does not establish that an arbitrary shell command completed.

The first useful slices are independent:

• Cooperative channels: use existing Server.wait_for; PR server(feat[wait]) Bound ordinary channel waits #809 adds an optional bounded ordinary wait without changing its blocking default. Do not introduce a competing wait_for_channel alias. A released channel waiter proves the channel signal, not successful application execution; producer status needs its own protocol.
• Visible literal presence/absence: return the satisfying immutable observation and retain the last observation/original cause on failure. Bound capture/matching work and command lifetime, and distinguish no sample from empty text. Absence means absent in the selected sample.
• Future observed text/bytes: arm before triggering input, expose readiness, match across chunks and fail on source loss/gaps, missing targets, deadline or cancellation. Supply its own minimal private observation adapter if needed; Observe pane output as bytes with explicit subscription gaps #800 is a semantic owner, not a merge prerequisite.

Keep explicitly named sampled quiet/stable predicates separate from completion. Regex work needs an enforceable bound before it joins a public waiter contract. Pane process exit is another fact: retained dead-pane status may be read when explicitly configured, while a removed pane’s missed status is unknown. Changing remain-on-exit is a visible policy choice, and OSC completion requires admitted application/shell cooperation.

Do not use history_size + cursor_y as a lossless absolute anchor. PR #808 demonstrates substantial repeated output between identical capture/history/cursor observations. Native history counters may aid a qualified daemon/grid lifetime; unavailable or ambiguous evidence remains unknown. Cursor semantics belong to #740 and the ambiguity classification to #671.

The refreshed private owned-PTY consumer passes its 13 declared matcher/capture/group/reset assertions on current master with the recorded tmux 3.8a binary. Stronger production controls fail: cancelling a worker-thread command leaves its native client alive until controlled release, and a proposed injected engine can dispatch to daemon A while the private observer attaches to facade daemon B with the same numeric session ID. These failures remain recorded; successful matching alone does not qualify backend ownership or command cancellation.

No broader waiter PR is selected in this limited delivery. PR #594 remains a larger framework proposal. A standalone minimal waiter may use existing trunk primitives and private adapters, but must prove its own bounds, original errors, cleanup and borrowed-resource survival without an unmerged engine, deadline, snapshot or subscription dependency. PR #810 is a compatible proposed retained-value example, not a prerequisite. #584 owns richer retained failure evidence.

Historical prior art: cooperative MCP channels and capture-based MCP waits. The earlier proposal’s absolute-anchor claim is superseded by the counterexample above. Read-only locators, mouse/fill actions, complete graphs and wider application adapters remain follow-ups.

Track this under #629 and Python project 3; #456 records environment profiles, #689 owns native cancellation, #732 owns channel deadlines, and #803 owns ordered multi-pane operations. Preserve existing synchronous behavior and unknown submitted outcomes; never retry an uncertain accepted mutation. No macOS work, minimum-version change or upstream tracker write is included.