fix(robustness): graceful per-page detection + clearer errors (agent-team review)

[san64777]

Context

You asked: "if there's an image in a PDF it doesn't warn and skip it, it crashes - at least it should complete the next pages," and to re-review the code with the agent-team. This is the result of a real-world QA pass plus a 3-lens agent-team review (QA / PDF-engine / independent code review).

The reported bug (fixed)

detect() / make_fillable() raised ScannedPDFError on the first image-only page, discarding every other page. A mostly-vector form with one scanned/image page (a cover page, an exhibit) got zero fields. This violated the best-effort detection contract (best-effort detection must degrade gracefully).

Fix: detect_manifest now skips a scanned or error-throwing page with warnings.warn and processes the rest. ScannedPDFError is raised only when every page is image-only (so a genuinely scanned single-page doc still refuses - Gate C preserved). Verified: a vector/image/vector PDF now detects fields on the two vector pages and warns about the image page.

Other issues the review found (fixed)

• fill() with a list value on a non-multi-select field crashed with a cryptic pypdf AttributeError (a real bug in the guaranteed core, from the multi-select work) -> now a clear ValueError.
• _inherited could infinite-loop on a circular /Parent chain (malformed PDFs contain these) -> cycle guard added.
• read_fields aborted the whole read on one malformed widget -> now skips it with a warning, recovers the rest.
• flatten on a fieldless PDF raised a raw pypdf error -> now a no-op returning the input.

Deferred (tracked in D020, out of scope per rule 5)

Typed PDF-open error family (corrupt/encrypted input), rotated-page placement (needs golden-harness), build() duplicate-name collision, XFA-dropped warning, text/checkbox sub-pixel guard.

Validation

• 7 new regression tests (per-page skip, all-scanned refusal, fill misuse, cycle guard, malformed-widget skip, flatten no-op).
• Re-ran a hands-on QA probe: the mixed image/vector doc now completes; single-page image-only still refuses gracefully; fixtures unaffected.
• CI: ruff, mypy, 111 tests, license-gate, no-dashes all pass. Bumps to 0.3.2 (rolls up the held 0.3.1 list-box fix).