PNT1 1/8: radius and shadow paint engines (paint-radius.ts, paint-shadow.ts, paint-aa.ts), vectors and host plumbing

[thejackshelton]

PNT1 part 1 of 8 (T115; notes/T046-paint-spec.md §5.2): the radius and shadow paint engines. This is the reference code that the device runs translated, plus its host plumbing.

First of a stacked chain. Base: master. The later parts are stacked on review/<parent> branches. The chain lands as a PNT1 train after trains 1–4. It was split by theme from pnt1-shadow-v2 53ca9f7db2, whose sources parts 1–3 together equal.

What changed

• packages/layout/src/paint-radius.ts: border-radius geometry as Blink 145 paints it:
  • CalcRadiiFor resolves against the layout border box.
  • gfx::SizeF's clamp, then ConstrainRadii.
  • The padding-edge radii.
• paint-shadow.ts: box-shadow layers as Chrome composites them:
  • spread shapes and the blur mask;
  • each shadow onto its backdrop.
  • It runs on paint-aa.ts's port of Skia's analytic AA.
• Tests: paint-radius.test, paint-shadow.test and paint-aa.test.
• Paint vectors. The inputs come from scripts/gen-paint-inputs-radius.ts and gen-paint-inputs-shadow.ts, run by one paint-inputs regen step that has its .gitattributes lines. The harness gains its paint:radius and paint:shadow cases.
• Host plumbing:
  • ExpectedEngine.paint (PaintEngine) and the layout border-box size in the expected dump's node geometry;
  • SampleBox.size.
• Ports registry. docs/ports.json registers the cited Blink and Skia files. float_rounded_rect.cc's Adobe BSD notice is kept in paint-shadow.ts, and gen-third-party-notices.ts gets its title.
• Tool: packages/dragon/test/tools/check-pnt1-additive.ts, the PNT1 additive check.

Changed tests (reasons)

• layout/test/paint-seams.test.ts: the radius and shadow seams are now filled (FILLED, plus their index.ts export lines).
• parity/test/samples.test.ts, paint-seams.test.ts and native-compare.test.ts: their SampleBox literals gain the new required size field. No assertion changes.
• dragon/test/expected-dump.test.ts: the engine it builds passes the new paint field.

What passed

• pnpm regen: fixed point after 3 passes. The outputs are in their own commit, 804faeb.
• pnpm typecheck: clean.
• pnpm test: 3598 passed, 12 failed, 1 skipped.

The 12 failing tests are the device-record checks (lanes.test, lanes-records.test, device-failures.test, p6a-promotion.test, lanes-concurrent.test). They fail only because the committed packages/parity/out/lanes.json is host-only: the device lanes run under the device lease at this branch's turn in the PNT1 train. No device run has been made for this branch.

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