test: add end-to-end skewed-torus and synthetic holonomy Re(τ)<0 tests

Finding-H from doc/reviewer/external-audit-2026-05-30.md
(java-port-audit missing-test item 7).

Guards against a regression where compute_period_matrix reverts to
reduce_to_fundamental_domain (no mirror fold) instead of normalizeModulus
(Finding 6 fix, Java-faithful): such a revert would silently produce
Re(τ) < 0 for lattices where the natural generators give a negative
real part.

Two new tests in test_phase7.cpp:

1. SkewedTorus_ReTauNegativeBeforeNorm_FoldedToPositive
   - New mesh: code/data/off/torus_skewed_4x4.off
     Flat 4×4 torus on parallelogram lattice ω₁=(4,0) ω₂=(-1,4);
     16 vertices, 32 triangles, χ=0 (genus 1).
   - Full pipeline: newton_euclidean → cut_graph → euclidean_layout
     → compute_period_matrix(reduce=false) + compute_period_matrix(reduce=true)
   - Asserts: raw Re(τ) < 0, normalized Re(τ) ∈ [0,½], Im(τ) > 0, |τ| ≥ 1

2. SyntheticHolonomy_NegativeReTau_NormalizedToPositive
   - Bypasses mesh; supplies explicit ω₁=(4,0) ω₂=(-1,4) directly
   - Asserts raw τ = -0.25+i (to 1e-10), normalized τ = 0.25+i (to 1e-9)
   - Pin-points the mirror fold: Re(-0.25) → Re(+0.25)

277/277 CGAL tests pass, 0 failed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

code/data/off/torus_skewed_4x4.off

@@ -0,0 +1,50 @@
+OFF
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