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

[mbreissi]

Phase-5 D-U28 alignment

Retires the main instance sentinel for telemetry-processor. As a single-instance
component, all of its own UNS topics are now component-scope — the component token is
followed directly by the class, with no instance segment:

• .../telemetry-processor/main/state → .../telemetry-processor/state
• .../telemetry-processor/main/cmd/<verb> → .../telemetry-processor/cmd/<verb>
• .../telemetry-processor/main/metric/pipeline → .../telemetry-processor/metric/pipeline
• .../telemetry-processor/main/data/downsampled → .../telemetry-processor/data/downsampled
• .../telemetry-processor/main/evt/{severity}/{type} → .../telemetry-processor/evt/{severity}/{type}

Scope

• No functional code changed. The core library (../core/libs/rust, now D-U28) already
  emits these at component scope: gg.events()/data()/app()/uns(), the state
  keepalive, the messaging metric target, and the cmd inbox all resolve to
  .../telemetry-processor/{class} with no instance token. telemetry-processor just calls
  those accessors, so the wire topics change automatically.
• Docs + config examples + one code comment. Topic strings updated across README.md,
  docs/, recipe.yaml, and test-configs/config.json (so a user copying an example gets
  the D-U28 topic).
• Stale comments corrected. Comments claiming the events() facade is "bound to the
  main instance" are now accurate — it is component scope under D-U28.
• Verified: no inbound topic-parsing code assumes a fixed main/instance position; only
  comments/docs/config-example strings changed. The local-output identity restamp
  (with_instance(&route.id)) is unrelated — it stamps the data product's envelope
  identity, not the component's own topics.

Validation

• cargo check green from the sibling worktree against the D-U28 core dep.

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