Characterize orientation-predicate robustness over R² (tests for #1106)

[grootstebozewolf]

Description

Adds a test-only harness that characterizes the robustness of the orientation
predicate (Orientation.index → CGAlgorithmsDD) over arbitrary double
coordinates, which is the property the point-line orientation robustness issues
ask for. The headline tool is a counterexample hunter that compares a
predicate against a self-contained exact reference (BigDecimal, exact for
dyadic doubles) and searches adversarial inputs for sign disagreements.

Key findings (all backed by passing tests):

• In-band soundness. Orientation.index agrees with the exact sign on
  every one of millions of adversarial cases — uniform-random across
  magnitudes, near-collinear at ULP-level perpendicular offsets, and the
  double-precision-hard cases from OrientationIndexFailureTest — for
  coordinates roughly in [2^-511, 2^511].
• The hunter is not vacuous. Over the same inputs it finds 10^4–10^6
  counterexamples for the predicates DD replaced (RobustDeterminant,
  NonRobustCGAlgorithms), while DD yields zero.
• Range limit (real counterexamples). Outside that band DD fails trivially:
  for |coord| ≥ 2^512 the products overflow (Inf − Inf = NaN) and for
  |coord| ≲ 2^-512 they underflow, so orientationIndex returns 0
  (collinear) for points that are not collinear. e.g.
  (0,0),(1e200,1e200),(2e200,2.0000001e200) is CCW but DD reports collinear.
  A robust predicate would scale coordinates to avoid this.
• Latent defect found (test-source only). While property-testing the
  Shewchuk expansion primitives, ShewchuksDeterminant.fast_expansion_sum_zeroelim
  was found to double-count the first component and drop the largest for inputs
  of length ≥ 2 (post-increment e[eindex++] where Shewchuk's reference uses
  pre-increment e[++eindex]): e.g. [1,16,256,4096]+[2,32,512,8192] → 822
  (should be 13107). This is masked in orientationIndex by the Stage-A float
  filter (verified exact over 2M near-collinear cases) and lives in test
  sources only; it is documented in code, not auto-fixed.

Relevant issues

• Summary: Point-Line Orientation robustness issues locationtech/jts#1106 — Summary: Point-Line Orientation robustness (umbrella)
• Orientation.Index problem with COLLINEARity locationtech/jts#750 — Orientation.index problem with collinearity
• Change Shewchuk Orientation filter to Ozaki et al. filter. locationtech/jts#1093 — discussion of solutions
• Covers predicate incorrect for Line/Point due to geometric robustness libgeos/geos#968, ST_Intersection give results that are inconsistent with ST_Distance libgeos/geos#1207, RelateCompute intersection report inconsistent for lines and boundary points libgeos/geos#1064 — related downstream symptoms

Contribution type / module

• Type: enhancement (tests / characterization)
• Module: jts-core (test sources only — no production code changed)

What's included (all under modules/core/src/test/.../algorithm/)

• RocqRefRunner + RocqRefRunnerTest — exact orientation reference and
  soundness tests: exhaustive over integer coords |c| ≤ 4, plus large
  randomized / near-collinear / boundary samples over |c| ≤ 2^25 (exact in
  64-bit), and the R² exact-BigDecimal reference; an optional loader for
  externally-exported reference vectors.
• DDCounterexampleHunter + OrientationDDRobustnessTest — the hunter,
  adversarial generators, and the overflow/underflow characterization tests.
• ShewchukExpansionExactnessTest — exactness of Two_Sum / Fast_Two_Sum /
  Two_Product, end-to-end exactness of ShewchuksDeterminant.orientationIndex,
  and the documented fast_expansion_sum_zeroelim defect.

All new files carry the EPL/EDL license header and are JUnit tests.

Notes

• This PR targets the fork's master, not upstream; commits are not ECA
  signed-off.
• The exact reference is self-contained. As an aside, its verdicts (including
  the overflow/underflow counterexamples) were independently spot-checked
  against an external GMP-backed exact-integer orientation oracle and agreed in
  all cases; that oracle is corroboration only and is not vendored or required.

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