docs(d-u28): component-scope UNS topics (retire main)

[mbreissi]

Phase-5 D-U28 alignment — file-replicator

Aligns file-replicator with the org-wide D-U28 UNS change: the instance token is now optional and the main sentinel is retired. Component-level traffic is component-scope (no instance segment); per-instance events keep their real component.instances[].id.

Docs + code-comment accuracy only — no functional code changed. cargo check is green.

What changed

• Dropped /main from every UNS topic string: command inbox (…/cmd/#), state keepalive (…/state), component metrics (…/metric/{name}), …/cmd/set-activation, …/cmd/get-status.
• Reworded ("main" instance) / (main instance) doc-comment phrasing → (component scope, no instance token) in src/{app,control,events}.rs.
• Rewrote the narrative command-inbox line in docs/reference/messaging-interface.md: component-scope inbox, instance token optional and present only for explicit multi-instance addressing.
• Made the UNS grammar show the instance segment as optional ([{instance}/]), and corrected the fleet-consumer subscription example for the new level count (component-scope state/metric/component-level evt lose the instance +; a second +/evt/# line still catches per-instance events).

Files

• DESIGN.md — command-inbox topic string
• docs/how-to-guides.md — set-activation topic, metric topic, grammar + fleet-consumer subscriptions, "command inbox" wording
• docs/reference/configuration.md, docs/reference/metrics.md — metric topic
• docs/reference/data-types.md — get-status topic
• docs/reference/messaging-interface.md — grammar, {instance} bullet, command-inbox narrative, state + metric topics
• docs/tutorial.md — "component-scope command inbox" wording
• src/app.rs, src/control.rs, src/events.rs — doc-comment/comment rewording + topic strings

Verification & a gap to flag

• No topic-parsing code touched. file-replicator delegates all topic handling to the library command inbox (gg.commands()) / event facades (gg.instance(id).events() / gg.events()); the old hand-rolled parse_cmd router is already deleted. Every reference edited was a comment/doc/string.
• Dependency gap: Cargo.toml pins edgecommons to git rev 1c01ace0, which is an ancestor of the D-U28 merge and does not include it — so the built component still emits the main sentinel today. These docs now lead the wired behavior. tests/p3_control_emqx.rs:396 asserts identity.instance() == "main" and is correct against the pinned pre-D-U28 core; it will need to flip to the no-instance value once the rev is bumped to a post-D-U28 commit. Bumping the rev + updating that test is a wire change requiring the interop matrix + lab regression, so it is intentionally not in this docs PR.

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