W10 of SEAMLESS-UPDATES. Fixes BigscreenVR/spt-bs-releases#341 and fixes BigscreenVR/spt-bs-releases#256.

Operator ruling (2026-09-24): instance state decides which instance a bare id reaches and may require a wake first. It never gates a delivery window.

## What changes
- **The AND is deleted.** Both hook drains (`api poll`, `api worker-poll`) filtered an include-deferred poll through `resting::deferred_held` (dormant or suspended). The window depends on activity; the rest record doesn't carry activity, so it can't answer a window question. The function is removed along with its test. A dormant endpoint now drains its busy-window messages on its own poll. So the away notice arrives while the agent is away, and the return notice can only come after it. A suspended endpoint gets them on its first poll after waking.
- **`--active-only` becomes `--busy-only`.** `--active-only` and `--deferred` still parse as hidden aliases. `api poll --include-deferred` and the `deferred=1` column keep their names.
- **REQ-INST-6 is reworded in place.** Its title stated the rule this PR deletes. It now covers the rest record and its remote suspend/wake, and points to REQ-DELIVERY-WINDOW-IS-ACTIVITY-AXIS-ONLY for delivery windows.
- **Docs:** CONTEXT.md replaces the deferred-message gate clause with what does happen, and names dormant's two arms. The messaging, instances and lifecycle docs are updated, plus CHANGELOG. reference.md and changelog.md are regenerated.
- **Tests:**
  - The drain unit tests now assert delivery, in order, while dormant and after waking from suspended.
  - A new flag test covers `--busy-only` and both hidden aliases.
  - A new integration test, `busy_window_axis_e2e`, drives the real away/return notice producer and the real binary, and asserts ORDER. It's classed HEAVY at birth in both HEAVY strings.

## Evidence (hfenduleam, Windows)
- check, clippy `-D warnings`, `xtask check` and treqs all pass.
- Targeted unit tests: 49/49 and 18/18.
- The integration test passes. With the old gate put back, it fails at its tagged assertion (`busy_window_axis_e2e.rs:170`, left 0 right 2), and so does the drain unit test (`delivery.rs:1507`).

This PR's CI run covers Linux.

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