loader: the plugin observer, host series and intake are the dispatcher's (door-only)

[MattJackson]

CONTRACT-ABI lane, item 3 (the door-only static seams, part 2): the plugin observer and the host's series predicate (coordinator ruling 2026-10-14 (a)).

door-only static-seam rule (docs/design/BUSBAR-1.6.0.md l.1475-1478 R6, l.1543): no static fn-pointer seam installed into the binary closure; exactly the two oauth2/admin seams are exempt. qa/door-only.toml rows: crates/plugin-loader/src/observe.rs OBSERVER (OnceLock<&'static dyn PluginObserver>) and HOST_SERIES (OnceLock<fn(&str) -> bool>). The composition root installed Rust code into both, and the INTAKE they fed was process-global with them.

What moves:

• observe::Observing (new) holds the observer, the host's series predicate and the bounded intake (INTAKE_BOUND, never blocking, drops counted). Installs stay first-wins and one-way, as before.
• The Dispatcher owns one (Env::observing; Dispatcher::observing()). An Adopter hands it to every door the dispatcher binds (Adopter::observing), and EnvelopeObserver carries it, so a door's envelope folds through its own dispatcher's observability. A door with no dispatcher (a test adopter) gets an empty one and drops, which is the old "no observer" state.
• The export door's shed fold reads the observability off its instance's dispatcher, and ExportRows::open's series grant reads it off the axis's dispatcher (grant_series(observing, …)).
• The composition root installs its observer (root::observe::install) and its series predicate (root::linked::register_exports) on the process's one dispatcher (root::dispatch::dispatcher()), which main builds before both.
• qa/door-only.toml strikes both rows.

Behaviour is unchanged. Every door already binds on that one dispatcher: the transport doors through root::doors::install_dispatcher, and the plane probes through door_probe_dispatcher(), which returns it.

Tests:

• In this crate's test binary every Observing stands with the shared recorder, so the fold-log tests and the export conformance both-folds proof (an_opened_sinks_envelope_reaches_the_host_observability, a_first_party_series_is_granted_through_either_door_and_to_nobody_else) read as before. The latter now names its host series on the dispatcher its opens bind on. settle() waits on every intake the binary started.
• RED: observe_tests::the_observer_and_the_host_series_are_the_dispatchers. A series predicate installed on one dispatcher refuses a first-party claim on the host's series there and not on another dispatcher; the second install is refused; an adopter's observability is its own dispatcher's. It does not compile on predev.

Shared files (rule 20/23): merge-tree is clean against every open PR touching these files: lane-proof-abi-mac, #1004, lane-a2a-push-ca, lane-law0-root, #992, #986, #971, #961, #960, #953, #952, #893, #865 and #864. It conflicts only with the drafts #778 and #561.

No dependency change.

[MattJackson]

Rule 20 note from #1031 (row 155, draft): one disjoint line in root/linked.rs (hot_answer_inline's cap reads busbar_kernel::proxy::max_translate_body_bytes). A test merge is clean.