gamut-cli: report a C2PA manifest store in gamut inspect, and state that it is not validated

[justin13888]

Part of #239 — one of its acceptance criteria, with no child issue until now. Blocked by #428 and
at least one container slice (#429 is enough).

Goal

gamut inspect reports a C2PA manifest store's presence, size and byte offsets, and states
plainly that it does not validate it.

Scope

gamut-cli is the sandbox that exercises implemented features, and Command::Inspect
(src/main.rs:66) already exists. This adds one section to its output, for every format whose
crate can locate a store — HEIC first (#429), then each container slice as it lands (#440,
#442, #444, #445, #446).

What to print

• Whether a store is present.
• Its size in bytes and its half-open byte range in the file.
• For BMFF, the box_purpose of the carrying uuid box (manifest / original / update) — a
  file mid-update legitimately carries two stores, and hiding that behind "1 store found" is
  misleading.
• An explicit disclaimer. Not a footnote: the line that reports the store says gamut does not
  check the signature, the hash binding, or the trust list, and names c2pa-rs as what does.

Why the disclaimer is a requirement and not politeness

A tool that prints "C2PA: present" beside EXIF and ICC reads as verified to anyone who has seen
a Content Credentials badge. gamut deliberately holds no crypto and reaches no verdict (#239's
scope), so the output must not let a reader infer one. Phrasing that implies validation is a
defect in this issue, not a wording preference.

What not to do

• Do not print the store's contents, decoded or hex-dumped by default. It is opaque bytes to
  gamut, it is routinely tens or hundreds of kilobytes once a manifest embeds a thumbnail, and
  rendering it invites exactly the "gamut understands manifests" reading above. A byte range is
  enough to hand to c2pa-rs or to dd.
• Do not report a remote-provenance URL as a store. That is gamut-xmp/gamut-metadata: report remote provenance from dcterms:provenance #449, the XMP
  dcterms:provenance criterion; none and remote are different answers and both differ from
  embedded.

Note on gates

gamut-cli is excluded from the coverage gate via --ignore-filename-regex, so the reporting
logic should live where it can be tested — in the format crate or a small helper — with the CLI
holding only the formatting.

Reference

C2PA Technical Specification 2.4 §15.12 (validation is a validator's job), §A.5.3 (box_purpose).
Staked by #427.