feat(api): content bleed — sections fill to the trimmed page edge

[DemchaAV]

Why

A section's background fill stops at the page content margin. Full-bleed chrome — a masthead band reaching the top and side edges, an edge-to-edge accent strip — was only reachable with a hand-computed negative-DocumentInsets trick that also drags the section's children off the page, and silently no-ops a negative bottom (the vertical flow swallows amount <= EPS in advanceSpace). There was no first-class way to say "this fill reaches the page edge."

What changed

• New DocumentEdge enum + DocumentBleed record in document.style, mirroring the DocumentLineCap / DocumentInsets idioms (none() / all() / of(...)).
• AbstractFlowBuilder.bleed(DocumentBleed) and bleedToEdge(DocumentEdge...) — section/flow builders declare which edges bleed.
• Additive DocumentNode.bleed() default (none()), so every other node is unchanged; SectionNode carries the bleed (back-compat constructor preserved).
• LayoutCompiler.compileComposite resolves the decoration box against the page canvas and relaxes the vertical content-area clamp on bled edges. Only the fill/border bleeds — the section's children stay inside the content margin, so a heading never runs off the page (an improvement over the negative-margin trick, which moved children too).

Lane: canonical public DSL + shared-engine (one compile method). The none() path is byte-identical — a static instance, no hot-path allocation.

Scope: section bleed. Line/leaf bleed (a line's geometry is fixed by node.startX()/endX(), not the placement width, so it belongs with LineBuilder.fill()) and migrating the existing MintEditorial / MintEditorialLetter masthead off the negative-margin idiom are deliberate follow-ups.

Verification

• ./mvnw test -pl . — 1487 tests, 0 failures, 0 visual baselines changed (proves the non-bled path is byte-identical).
• DocumentBleedTest: a bled section's fill reaches x=0 / width=pageWidth / top=pageHeight; a non-bled section stays inside every margin; DocumentBleed factory unit checks.
• Runnable BleedExample (full-bleed masthead + edge-to-edge band) with a committed preview (assets/readme/examples/content-bleed.pdf) and an examples README row; CHANGELOG entry under v1.9.0.

@since 1.9.0 on all new public API.