docs(java-parity): P2/Matrix jReality notes + Phase 9c dependency warning

- Infrastructure table: add Matrix (→ Eigen inline, done) and P2 rows.
  P2 marked "port inline with Phase 9c" — almost exclusively consumed by
  FundamentalPolygon/CanonicalForm/SurfaceCurve (all Phase 9c), so building
  p2_geometry.hpp as a standalone ahead of 9c would be dead weight.
- Phase 9c rows in the utility table: ⚠️ annotation listing the exact P2.*
  methods needed (makeDirectIsometryFromFrames, projectP3ToP2,
  imbedMatrixP2InP3) plus the P2Big/cpp_dec_float_50 high-precision
  prerequisite for the group-relation product ∏gᵢ = Id.

Ensures a future Phase 9c session starts with full context and does not
discover the jReality/precision dependencies mid-port.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

doc/roadmap/java-parity.md

@@ -52,6 +52,8 @@ depend on.  Porting them once unblocks all downstream consumers.
 |---|---|---|---|
 | **`de.jreality.math.Pn`** | Projective-metric primitives: inner product, norm, distance, geodesic interpolation in Euclidean / Elliptic / Hyperbolic signatures | ⚠️ **partial** → `pn_geometry.hpp` (2026-05-30) | 6 functions (~30 LOC) cover the entire surface used by the Java algorithmic core (27 files, ≈106 imports). Unblocks: HyperbolicLayout, SphericalLayout, FundamentalPolygon (9c), KoebePolyhedron (10c'), quasi-isothermic (10e), hyperelliptic θ, CircleDomain (11c). |
 | **`de.jreality.math.Rn`** | Real vector / matrix utilities (add, sub, scale, dot, norm, …) | ⚠️ partial → Eigen | The Rn surface used in the core (16 distinct methods) maps directly onto Eigen VectorXd / MatrixXd operations. No separate port needed; call-sites replace `Rn.foo(a,b)` with Eigen expressions inline. |
+| **`de.jreality.math.Matrix`** | 4×4 matrix wrapper (times, print) | ✅ → Eigen `Matrix4d` | `Matrix.times` = matrix multiplication; call-sites replace it with Eigen `operator*`. No separate port needed. |
+| **`de.jreality.math.P2`** | Projective plane geometry: `lineFromPoints`, `pointFromLines`, `projectP3ToP2`, `makeDirectIsometryFromFrames`, `imbedMatrixP2InP3`, `chopConvexPolygonWithLine` | ⚠️ **port inline with Phase 9c** | Used almost exclusively by `FundamentalPolygonUtility` + `CanonicalFormUtility` + `SurfaceCurveUtility` (all Phase 9c). Port these ~5 functions as a `p2_geometry.hpp` at the start of the Phase 9c session, not before. `lineFromPoints` / `pointFromLines` = cross products in ℙ²; `makeDirectIsometryFromFrames` = direct isometry from two point-pairs (~25 LOC). |
 | **`de.jreality.math.MatrixBuilder`** | Isometry construction: `rotateFromTo`, `translateFromTo` in Euclidean / Hyperbolic models | ❌ not yet | Used only by `HyperIdealHyperellipticUtility` (hyperelliptic θ). Port on demand when that utility is needed. |
 | **conformallab XML types** (`de.varylab.conformallab.types.*`) | JAXB-generated persistence format for Java GUI state (`HalfedgeEmbedding`, `UniformizationData`, …) | ❌ not planned | GUI / persistence layer, not an algorithmic substrate. conformallab++ has its own serialisation (`serialization.hpp`). Asset-specific parsers (e.g. for `lawson_curve_source.xml`) are added on demand, not as a general port. |
 
@@ -66,8 +68,8 @@ They are candidates for Phase 9 or Phase 10.
 |---|---|---|
 | `ConesUtility` (~200 lines) | Prescribed cone angles Θᵥ ≠ 2π — Euclidean mode only | 9d.1 |
 | `CPEuclideanFunctional` | Face-based circle-packing energy (BPS 2010) | 9a.1 |
-| `FundamentalPolygonUtility` (698 lines) | Construction + canonicalisation of 4g-gons for genus-g | 9c |
+| `FundamentalPolygonUtility` (698 lines) | Construction + canonicalisation of 4g-gons for genus-g. **⚠️ jReality dependency:** uses `P2.makeDirectIsometryFromFrames`, `P2.projectP3ToP2`, `P2.imbedMatrixP2InP3` → port these as `p2_geometry.hpp` at the start of the Phase 9c session (see Infrastructure table above). Also uses `P2Big.*` (high-precision counterparts) + requires `cpp_dec_float_50` for the group-relation product ∏gᵢ = Id. | 9c |
-| `CanonicalFormUtility` (532 lines) | High-level wrapper for 9c — drives canonicalisation pipeline | 9c |
+| `CanonicalFormUtility` (532 lines) | High-level wrapper for 9c — drives canonicalisation pipeline. **⚠️ jReality dependency:** same `P2.*` methods as above via `FundamentalPolygonUtility`. | 9c |
 | `CuttingUtility` + `SurgeryUtility` (~800 lines) | Mesh cuts and gluing operations needed for fundamental domains | 9c (foundation) |
 | `DiscreteHarmonicFormUtility` (657 lines) | Discrete harmonic 1-forms via cotangent Laplacian (Hodge theory) | 10a prerequisite |
 | `DiscreteHolomorphicFormUtility` (285 lines) | Holomorphic differentials via Mercat complex structure | 10a (Bobenko-Springborn 2004 §6) |