`extension_points` is an `[app]`/`[group]` node key. Placed at top level
(before any table header) TOML reads it as a root key — and `Manifest`
silently dropped unknown root keys, the same silent-drop class that bit in
C5 (a key absorbed under `[group]` and discarded). Add
`deny_unknown_fields` to `Manifest`, `ManifestApp`, and `ManifestGroup` so a
misplaced or typo'd key is a hard parse error instead of a no-op apply.
Regression test covers top-level placement, an in-`[app]` typo, and the
correct node-key form.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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