[FEATURE] Deprecation-manifest codegen target that emits changelog fragments in diff mode

Sub-issue of #639 — surface 3 of 3 ("What flows where"), automatic path. Stretch.

Depends on #674 for the extraction carrier and on #675 for the deprecation fragment type.

Scope

Detect when a deprecation is introduced, so nobody has to remember to write the fragment.

A codegen deprecation-manifest target emits a small structured list of every deprecated element — field path or model name, plus its message. Think of it as a generation target simpler than the Markdown reference: same extraction input, a summary instead of a document.

Run in diff mode against the last released tag, the target compares that manifest to the current one and emits each newly-appeared entry directly as a deprecation fragment. Detection and scaffolding fold into the one target, and the fragment body is the message already authored on the annotation — nothing is written twice.

Open questions

• Is manifest-plus-diff the right mechanism, or does a simpler check over the diff of the models themselves do the job?
• Where does diff mode run — a release-time tool, or a CI check on every PR that fails when a new deprecation ships without a fragment? The second catches the omission when it is cheap to fix.
• What is the manifest's stable identity for a field, given fields move between models? A qualified path is the obvious answer and needs confirming against the union/arm cases.

Acceptance

• The manifest target emits one entry per deprecated field and model, with its message.
• Diff mode against a tag with no deprecations emits a fragment per deprecated element; against the current HEAD it emits nothing.
• A deprecation removed between releases does not emit a fragment.

Integration points

• A new codegen output format alongside markdown, reading the same FieldSpec.deprecated / model-spec carrier from #674.
• Fragments land in the affected package's changelog.d/ as <pr>.deprecation.md.