docs: document API-component and semantic-header triage scope; add Security to user-manual nav

[oscerd]

Context

The ASF Security Team (Piotr) asked us to make the project security model more
prominent for reporters and to clarify two recurring triage questions so future
reports are handled consistently. This PR is the apache/camel side; a
companion PR against apache/camel-website adds a prominent "Security model and
report scope" section at the top of https://camel.apache.org/security/ linking
the canonical model page.

Changes

security-model.adoc — two triage clarifications folded into the model:

1. Camel-prefixed API-component headers (CamelFhir.*, CamelBox.*,
   CamelOlingo4.*, CamelAs2.*, ...) reaching an API producer from an
   untrusted source. Documented position: not always out of scope — it is
   conditional, and the condition is at the inbound consumer, not the API
   producer. These prefixes live in the Camel* namespace, so the protecting
   boundary is the inbound consumer's case-insensitive Camel*
HeaderFilterStrategy (the CVE-2025-27636 family mechanism). Added to the
   Camel-header / bean-dispatch abuse in-scope class.
2. Non-prefixed semantic headers a component reads (To, Cc in
   camel-mail, ...). Documented position: one uniform rule — framework
   auto-filtering is scoped to the internal Camel* / org.apache.camel.*
   namespace only; the rest is the route author's responsibility and a
   per-component documented contract; the inbound-filter class
   (e.g. CVE-2026-33454) and path-traversal class (e.g. CVE-2018-8041) still
   apply independently. Added as a Known limitations bullet.

nav.adoc — discoverability: the Security and Security Model pages were
reachable only by direct link. Added a top-level Security group to the
user-manual left navigation.

Notes

• Docs-only change; no code, no generated files affected.
• The triage positions were reviewed and confirmed with the operator before
  being written into the model.

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Claude Code on behalf of Andrea Cosentino

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