feat(pdf): debug node-label overlay - PdfDebugOptions with guides + semantic path badges

[DemchaAV]

What

DebugMode grows a second overlay: semantic node labels. PdfDebugOptions (guides + node labels + label-text mode) configures the canonical PDF backend via GraphCompose.document(...).debug(...), DocumentSession.debug(...), or PdfFixedLayoutBackend.builder().debug(...).

With nodeLabels() enabled, every rendered node prints its stable semantic path — the same path layoutSnapshot() reports — once per node and page, as a small corner badge straddling the top-right edge of the node's union bounds (5pt base-14 Helvetica on a pale halo; non-WinAnsi name characters degrade to ? instead of failing the render). Spot a misplaced block on the sheet, read its badge, grep the name in your builder code.

• LabelText.NAME (default) — compact own segment (PriceSummaryTitle[0]); FULL_PATH — whole ancestry.
• guideLines(boolean) everywhere became sugar over the options object; node-label settings survive the toggle.
• Debug overlays draw strictly on top of content, never touch measurement/pagination; disabled output stays byte-identical (covered by a same-length test, existing snapshots untouched).

Plumbing

• New DebugOverlayExample (features/debug) registered in GenerateAllExamples + ShowcaseMetadata (new "Debug & Diagnostics" group), examples README row + section, committed preview assets/readme/examples/debug-overlay.pdf (1.7 KB).
• CHANGELOG v1.8.0 → Public API entry.

Verification

• 13 new tests: PdfDebugOptionsTest (value semantics incl. null rejection), PdfDebugNodeLabelsTest (PDFTextStripper-based: NAME/FULL_PATH modes, off-by-default, guides-only isolation, Cyrillic Шапка → ?????[0] degradation, disabled-options length identity), DebugNodeLabelsDemoTest (visual diff vs plain render > 500 px).
• Full gate: ./mvnw verify -pl . — 1233 tests, 0 failures, javadoc green.
• Badge placement tuned against the rendered PDF over two iterations (top-left covered content start; final corner-badge straddles the top edge so only an ascender half-strip can be covered).