Scanned/ImageBased page_signals empty case untested

## Problem

The Full-mode process result early-return for Scanned/ImageBased PDFs produces an empty `page_signals` list (documented contract at src/lib.rs:193: "empty for DetectOnly and Scanned/ImageBased results"), but no test exercises that branch: no image-backed fixture exists in tests/fixtures, and the only empty-page_signals assertion is the DetectOnly test.

Analyze mode is covered (test_process_pdf_mem_analyze_mode_page_signals); the Scanned/ImageBased branch is not.

## Suggested fix

Add a Scanned/ImageBased fixture test asserting `page_signals == []` for a scanned/image-backed PDF in Full mode, or document the coverage gap explicitly.

## Evidence

- src/lib.rs:3704 -- `if matches!(pdf_type, PdfType::Scanned | PdfType::ImageBased) { return Ok(PdfProcessResult { ... page_signals: Vec::new(), ...}) }`
- tests/integration_tests.rs -- only DetectOnly asserts `page_signals.is_empty()`

## Source

Branch `feat/expose-detector-signals` at `cf0fe17` (PR pending).

## Metadata

Severity: P2
Confidence: 100
Reviewer(s): testing
Finding ID: src/lib.rs:193 + normalize(title)

Start with the Scanned/ImageBased early return at src/lib.rs:3704 and the existing integration coverage in tests/integration_tests.rs, especially test_process_pdf_mem_analyze_mode_page_signals. Find or add an image-backed PDF fixture, run the Full-mode process test, and verify that the result has an empty page_signals list.

Written by the indexing model from the issue text.