Add Phase 4 stub classifiers for Watermark and Formula block kinds.
Full detection deferred to Phase 7 per plan section 4.4 (line 1709)
and 4.6 watermark note (line 1752).
Changes:
- Create crates/pdftract-core/src/layout/watermark_formula.rs with
classify_watermark() and classify_formula() stubs returning false
- Update crates/pdftract-core/src/layout/mod.rs to export the stubs
- Add comprehensive module documentation linking to Phase 7 research
Acceptance criteria:
- BlockKind::Watermark and BlockKind::Formula variants exist (pre-existing)
- classify_watermark always false
- classify_formula always false
- No v0.1.0 block has kind=Watermark or Formula
Refs: pdftract-3jekw
