fix: CIRCULARSTRING(A,B,C,A) is a valid geometry (annulus)

[grootstebozewolf]

Fixes V-CS leftover after #86/#87.

topic: curve
claimId: V-CS
witness: circularstring-abca
macro: V-CS
lane: validity
chunk: closed-4
issue: #86

Catalogued slice

TRIAGE / EPIC Phase 2 V-CS (CircularString validity / simplicity).
#87 already made the 3-click shell CIRCULARSTRING(A,B,C,A) a full
circumcircle (draw / length / disc kit). This pin is isValid / parse
acceptance of that 4-control form, plus the concentric pair as a
valid annulus. ISO/IEC 13249-3. No DOI.

Not #114 / tb-h-1 paint (PRs #118 / #119, HOLD UX SIGN). Not complementary
sweep-mid for major-arc holes. Not H-CC area 50 + 12.5 acos(0.6).
Not 67-c / S15l / Touches-vs-Share. Not NTS. Not GEOS.

Observed

ISO/SFA wants an odd CircularString control count (3, 5, …).
CIRCULARSTRING(A,B,C,A) is 4 points: first arc A–B–C, then C–A has no
stored mid. WKTCircularStringTest.testRejectsEvenPointCount treated
every 4-point string as parse-invalid. After #87 the closed 4-control
form is a first-class full circle; GeometryCombiner already accepted
it, but CircularString.isValid() and CurveWKTReader did not.

Expected

• CIRCULARSTRING(-5 0, 0 5, 5 0, -5 0) parses, stays CircularString,
  4 controls, first=last, isValid() == true.
• Open leftover CIRCULARSTRING(0 0, 1 1, 2 0, 3 1) still fails parse
  and isValid().
• Odd count ≥ 3 unchanged (still valid).
• Concentric pair
  CURVEPOLYGON (CIRCULARSTRING(-5 0, 0 5, 5 0, -5 0), CIRCULARSTRING(-3 0, 0 3, 3 0, -3 0))
  is a valid annulus (shell + hole, both 4-control). WKT is not rewritten;
  no invented 5th control.

Change

• CircularString.isValidControlCount / isValid: empty or odd ≥ 3, or
  closed 4-control with a circumcircle (threePointCircleCloseMid).
• CurveWKTReader.readCircularStringText uses the same predicate
  (constructor still allows an invalid object; parse rejects open even).
• Tests: CircularStringValidTest, WKTCircularStringTest accept closed-4.
• README: odd-count rule is now enforced, with the [visual-qa] 3-click CurvePolygon circle is CIRCULARSTRING(A,B,C,A) but draws a closing line #86 exception named.

Tests (local)

mvn -pl modules/curve -Dcheckstyle.skip=true -Dpmd.skip=true \
  -Dtest=CircularStringValidTest,WKTCircularStringTest,CurveShapeWriterCurvePolygonTest,CircularDiscOverlayTest,CurveAwarenessGreenMetersTest test
# 69 tests, 0 failures
mvn -pl modules/curve -Dcheckstyle.skip=true -Dpmd.skip=true \
  -Dtest=WKT*Test,CircularStringToLinearTest test
# 73 tests, 0 failures

Checks

Checks: 4/4 green @ fce5d93

Merged to feature/sfa-curve-rgr by grootstebozewolf at 2026-08-23T05:26:22Z (1e446de7). Merge-in only; V-CS blobs unchanged from 2b56b1a4. NTS #19 / Proofs locationtech#532 do not need a SHA remint.