feat: lower purely-linear FlatZinc floats to LP-only continuous columns

[rasros]

Part of the hybrid MIP/CP engine (#1232). Stacked on #1294. Extends the LP-only continuous columns to the FlatZinc frontend (Phase 6's second half for FlatZinc).

What

A whole-problem gate: when every float in a FlatZinc model can be LP-only — no var float array, no nonlinear float constraint (float_times var·var, float_abs, array_float_element, float_min/max), no strict (float_lt/float_lin_lt), no ne, no reified float constraint, and no float objective — all floats lower to LP-only continuous variables (real columns the simplex resolves) instead of bucket-index integers. Otherwise the model keeps the existing bucketing unchanged. This avoids mixing the two representations in one problem.

• Prepass floatsAreLpOnly() sets the gate; allocFloat then allocates a real var (parallel realLo/realHi, passed to Problem).
• The linear float handlers (float_lin_le/eq, int2float, float_eq/le) emit real Linear rows over the real columns with raw double coefficients (no bucket scaling); the nonlinear handlers are never reached under the gate.
• Solution rendering (FlatZincWriter, OznApplier) reads an LP-only float's value from Sample.reals.

Robustness fix (benefits the whole system)

Surfaced that exactBasisFeasible declines on a degenerate inequality-plus-equality vertex (feasible → UNKNOWN). Added exactPointFeasible: certify the float primal point directly when it snaps to an exact dyadic rational — verify every row (=/≤) and box bound exactly in 128-bit. Wired as a fallback in solveAndCertify, so a feasible dyadic solution is certified SAT even when the basis reconstruction is finicky.

Test

FlatZincFloatConstraintsTest (LP-only engages for a linear model + solves feasibly; a nonlinear constraint keeps bucketing), FlatZincParseTest/FlatZincFloatCompareTest updated for the LP-only path, LpContinuousColumnTest (degenerate vertex certified via the exact point check). Full :klause:jvmTest, :klause-bench:test, :klause:lintDocs green.

Remaining

The native schema Compiler (CompilerAssertions.assertFloatLinear) is a separate pure-linear float surface (its own floatMeta* tables) — tracked as a follow-up.