P2.5.6 H-SHELL-N-MIXED overlay kit (Option C, OV-P2 / Bar 2)

[grootstebozewolf]

Stacked on #25 (cursor/p254-tangent-stamp-917c @ 8d1546a). OV-P2 / Bar 2. Option C only. Do not stack on #27 (OverlayNG-for-circles / public SegmentKind is a dead-end draft). Do not merge this into #7. Does not merge #12 / #13 / #15 / #16 / #17 / #18 / #19 / #20 / #23 / #24 / #25 / #7. Does not push to feature/sfa-curve-rgr or master.

Sentence

P2.1–P2.5.4 climbed. N-SS is locked to Option C (CurveSegmentString in jts-curve, 0 core SegmentString edits). The noder already named MIXED as an edge (P2.2 / #17): H-SHELL-N-MIXED diameter (-1 0)–(1 0). Overlay still assertNull. This rung consumes that named edge and returns exact CAP / CUP / SUB / XOR.

Assembly rule

Shared-edge walk, not a discrete node pair. CurveSegmentNoder.nodes stays null (the overlap is an interval). edges names the chord. MixedOverlapOverlay walks each hole-free CompoundCurve shell from the run's ends, treats the overlap walk as the shared edge, and classifies the rest walk as entirely IN or entirely OUT. CAP / CUP are the inner / outer shells. SUB / XOR splice the two rests into a bite on the diameter (not a punch). A rest that weaves in and out (collinear overlap plus a hidden crossing) stays null. Nested / crossing half-discs stay on HalfDiscOverlay.

The arrangement-walk-on-null-nodes hypothesis was checked and not used as a general faces path: nodes==null can hide a real crossing that the collinear pair aborted (HALF_DISC × HALF_CROSSING_UPPER). The certified rest-walk is the closed form. No snap-rounding, no core SegmentString / HotPixel / noding.snapround, no densify-then-overlay flagged exact.

Not this rung (existing named nulls, unchanged):

• H-ANNULUS-TANGENT / CIRCLE_INT_TAN — P2.2 pinch
• TOUCH-ext — P2.2 kiss
• R-LL collinear — lineal, not this kit
• H-SHELL-HOLE-X / holed Geometry — P2.3/P2.4
• P2.5.4 TANGENT_LEAVE_ANGLE — N≥3 near-tangent stamp

Cells

Cell	Product
N=2 pair-kit lasers (R1.5, even-n, ODD, HOLE-*, H-SAME-CIRCLE)	unchanged laser; isApproximate()=false
P2.5.4 HALF_DISC × HALF_HANGING × STADIUM_ODD	unchanged named stamp TANGENT_LEAVE_ANGLE
H-SHELL-N-MIXED (HALF_DISC × ON_DIAMETER)	this rung: exact CAP/CUP/SUB/XOR; SUB/XOR is a shell bite, not a hole

Pins

• isApproximate()=false on MIXED CAP/CUP/SUB/XOR. A chordsaw fallback fails that assert.
• Areas: CAP = 2 + π/2, CUP = 12.5π, SUB/XOR = 12.5π − 2 − π/2, reverse SUB empty.
• P2.1–P2.5.4 pins stay bit-identical.
• Slack 15%. OverlayNGCurve never Curved. Java 8, Checkstyle, PMD (AvoidBranchingStatementAsLastInLoop).
• No new public API. Package-private in org.locationtech.jts.operation.overlayng.curve.
• No circular noder in jts-core. No Option B SegmentKind. No createCompoundCurve(CoordinateSequence) polyline lie.

What this is not

• Not P2.5.5 OverlayNG-for-circles (Option B contract, OV-P2 / Bar 2) #27 / Option B / public SegmentKind
• Not a merge into feature/sfa-curve-rgr / PR SFA/SQL-MM curve awareness: Curve rename, arc-aware operations, OverlayNGCurve ratchet (#1195 RGR series) #7 / locationtech master
• Not HP.4, TANGENT overlay, TOUCH-ext overlay, PLG, COV, clothoid, TestBuilder, ML.2
• Not P2.5.5 core SegmentString / Noder

Verify

mvn -pl modules/curve -am test -DfailIfNoTests=false -Dcheckstyle.skip=true -Dpmd.skip=true \
  -Dtest=CurveSegmentStringTest,CompoundCurveShellOverlayTest,CircularArcOverlayTest,CircularDiscOverlayTest,CircularDiscPolygonOverlayTest,CircularLineOverlayTest,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:

• CurveSegmentStringTest 24 / 0 (MIXED overlay + pair-kit faces; pinch / kiss / holed / P2.5.4 stamp unchanged)
• CompoundCurveShellOverlayTest 28 / 0 (testMixedOverlapIsSharedEdgeBite is the new laser)
• CircularArcOverlayTest 11 / 0
• CircularDiscOverlayTest 12 / 0
• CircularDiscPolygonOverlayTest 7 / 0
• CircularLineOverlayTest 7 / 0
• OverlayNGCurvePhase0Test 19 / 0
• OverlayNGCurveRatchetTest 50 / 0
• OverlayNGCurvePerfGateTest 84 / 0 at 15% slack (MIXED CAP + SUB added)
• bash dev/check-no-curved.sh OK
• Checkstyle 0 / PMD clean on jts-curve