Module Ir.Fp_algebra

The licensed floating-point algebra over raw lowered code (gh-ocannl-998): a member of the Rewrites tier, after Online_softmax.

The rewrites are identities over the reals that change IEEE results -- contraction of a * b + c into one FMA, reassociation (constants, subtraction chains, the multiplication/division interchanges, integer-power unrolling) and the zero identities that infinities, NaNs and the sign of zero contradict -- each family gated by its licence key (Low_level.Fp_licence, from fp_contract, fp_reassociate and fp_finite_math). They used to run inside Low_level.simplify_llc, after virtualization and again after scheduling, so contraction crossed inlining boundaries: an inlined producer y := a * b fused with its consumer's y + c while a materialized one rounded, and a placement flip changed results at uniform f32. On the raw lowering, one assignment at a time, the member never sees an inlined read, so OCANNL's own algebra is placement-neutral, and simplify_llc is exact everywhere it runs. The trade, taken on purpose: a fold that exists only after inlining (a consumer contracting its producer's product, a scalar constant node's value folding into a quotient) is lost. Backend compilers keep their own contraction licences (cc_backend_fp_contract, NVRTC's --use_fast_math), which this does not govern.

It runs after Online_softmax: contraction turns the value reduction o := o + w * v into fma (w, v, o), which the probability hoist does not match.