CC-NEST-ANNULUS: stamp mixed nest; lock two-arc disc as D4

[grootstebozewolf]

Stacked on cursor/d5-annulus-tangent-d7aa @ 7370303 (draft PR #13). Does not merge #12, #13, or #7. Does not push to feature/sfa-curve-rgr or master.

Sentence

Every circular pair that has a closed form has a kit. Everything else is a named miss and the chordsaw. We will not write a circular noder in this PR.

Choice: stamp, plus a D4 lock

CC-NEST-ANNULUS is a named green null, not a laser. Kit if both operands certify as discs; else stamp. No noder was written.

Stamp (this cell)

Fixture: horizontal stadium |x|≤2, |y|≤1 strictly inside CIRCLE_5.

CURVEPOLYGON (COMPOUNDCURVE (
  CIRCULARSTRING (-1 -1, -2 0, -1 1), (-1 1, 1 1),
  CIRCULARSTRING (1 1, 2 0, 1 -1), (1 -1, -1 -1)))

Not a re-run of CIRCLE_5 \\ CIRCLE_3, not H-ANNULUS-TANGENT, not HALF_SMALL. Mixed CompoundCurve (arcs + segments). 0 nodes.

Why every kit misses:

• R1.5 / D4: stadium is not a disc (centreRadius is null)
• R1.6: stadium is not a plain polygon
• R1.7 clip() is two-node only, so a 0-node nest returns null
• TwoShellClip never runs: only one operand is a mixed CompoundCurve shell

assertNull("CC-NEST-ANNULUS: …") on CircularDiscOverlay and CompoundCurveShellOverlay. Public SUB may chordsaw; that path is approximate, not a laser. Do not punch a stadium-in-disc hole.

Kit lock (already D4)

A CompoundCurve of two semicircle arcs vs CIRCLE_5 both certify via CircularDiscOverlay.centreRadius. That pair already takes nestedAnnulus. This PR does not invent a new noder or a general CompoundCurve annulus. It adds a kit-level assertNotNull plus CAP/CUP/SUB/XOR exactness (area 16π, isApproximate()=false) so the two-arc encoding cannot silently become R2.

Out of scope

• No new public API
• No circular noder
• No P2.1 / N-SS / P2.5
• No densify-as-laser
• Slack stays 15%

Verify

mvn -pl modules/curve -am test -DfailIfNoTests=false -Dcheckstyle.skip=true -Dpmd.skip=true \
  -Dtest=CompoundCurveShellOverlayTest,CircularArcOverlayTest,CircularDiscOverlayTest,CircularDiscPolygonOverlayTest,OverlayNGCurvePhase0Test,OverlayNGCurveRatchetTest,OverlayNGCurvePerfGateTest
bash dev/check-no-curved.sh
mvn -pl modules/curve checkstyle:check
mvn -pl modules/curve pmd:check

All green after this revision:

• CompoundCurveShellOverlayTest 24 / 0 (CC-NEST-ANNULUS stamp on R1.7)
• CircularArcOverlayTest 11 / 0 (same named null)
• CircularDiscOverlayTest 12 / 0 (D4 two-arc lock + CC-NEST-ANNULUS stamp + H-ANNULUS-TANGENT)
• CircularDiscPolygonOverlayTest 7 / 0
• OverlayNGCurvePhase0Test 19 / 0
• OverlayNGCurveRatchetTest 44 / 0 (covers two-arc disc is EEEE)
• OverlayNGCurvePerfGateTest 78 / 0 (15% slack)
• mvn -pl modules/curve test: 589 tests, 0 failures
• Naming-guard OK; Checkstyle 0 violations; PMD check on jts-curve succeeded