HP.2 CurveHotPixel in jts-curve (arc ∩ pixel)

[grootstebozewolf]

Stacked on #26 @ 1b06a57. Sibling of P2.5.5, not the 74-file blast.

HP.0 / HP.1 done. This is HP.2 only.

What CurveHotPixel is

A package-private pixel around one node in org.locationtech.jts.operation.overlayng.curve. The square / half-open / scale contract matches snap-rounding (width 1/scale, left and bottom closed, top and right open). Intersection is arc ∩ pixel in the squared metric: circle–square / arc–AABB in scaled space, then restricted to the sweep. Not the supporting chord. Not HotPixel.intersectsScaled. Not a densified polyline.

The first pin is the H-SHELL-N-ODD tangent at (0, 5) (documented scale 10, pixel width 0.1):

• HALF_DISC and STADIUM_ODD arcs that leave that node hit the pixel — one intersection node, not a chord fake.
• HALF_HANGING's circular member misses (the pixel sits inside that disc: d²_max < r²).
• The supporting chords (HALF_DISC diameter y=0, STADIUM_ODD cap y=4) miss. That is the lie intersectsScaled on a chord would tell.

This rung does not snap leave-angles and does not walk faces.

Never list (catalog)

• intersectsScaled on a chord called circular
• curvature-order walk (already refuted, HP.1)
• densify then SnapRoundingNoder flagged isApproximate()=false
• edit org.locationtech.jts.noding.snapround / core HotPixel.java
• start HP.3 / HP.4 / HP.5 / P2.5.5
• merge into feature/sfa-curve-rgr / PR SFA/SQL-MM curve awareness: Curve rename, arc-aware operations, OverlayNGCurve ratchet (#1195 RGR series) #7

What this is not

• Not HP.3 (snap leave-angles onto distinct grid headings)
• Not HP.4 (faces after snap)
• Not P2.5.5 / core HotPixel / core SegmentString
• Does not rewrite HP.1 (testCurvatureOrderWalksWrongRings, 18.42 vs pair-kit 13π+2)
• Does not rewrite the P2.5.4 stamp (testN3TangentStampsNull / missReason P2.5.4 tangent leave-angle)
• N=2 pair kits and N=3 nine faces unchanged

Verify

Same suite as #26. Slack 15%. OverlayNGCurve never Curved. All green after this revision:

• CurveSegmentStringTest 28 / 0 (testN3TangentStampsNull and testCurvatureOrderWalksWrongRings unchanged; three HP.2 pins added)
• CompoundCurveShellOverlayTest 27 / 0
• CircularArcOverlayTest 11 / 0
• CircularDiscOverlayTest 12 / 0
• CircularDiscPolygonOverlayTest 7 / 0
• CircularLineOverlayTest 7 / 0
• OverlayNGCurvePhase0Test 19 / 0
• OverlayNGCurveRatchetTest 50 / 0
• OverlayNGCurvePerfGateTest 82 / 0 (15% slack)
• Naming-guard OK; Checkstyle 0 violations; PMD check on jts-curve succeeded

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

No noder was written. Core HotPixel.java was not edited. HP.1 pin and P2.5.4 stamp unchanged. HP.3 / HP.4 / P2.5.5 were not started.