math: compile FP-free with BND_MATH_NO_FP (incl. the single header)

[NiceAndPeter]

Phase 2 of the math-engine plan — the FP-free guard

Lets the library (and the single header) compile with no hardware floating point and no <cmath>, leaving the always-present integer/CORDIC engine to serve the full bnd::math API. The public surface, output grids, and types are unchanged — only the compute backend differs.

What BND_MATH_NO_FP does

• Compiles the double (FP) engine and its #include <cmath> out entirely.
• Implied by BND_MATH_FIXED; auto-enabled when __STDC_HOSTED__ == 0 (-ffreestanding).
• All transcendentals are constexpr under it (the integer engine).

How it works

• cmath_double.hpp resolves the macro and gates <cmath> + the whole dbl engine behind #ifndef BND_MATH_NO_FP. The guard is at conditional depth, so the amalgamator keeps the <cmath> include inline under the same guard — the single header honors the FP-free path identically to the modular headers (no longer hoisted unconditionally to the top).
• Dispatch and the constexpr BND_MATH_FN macro in cmath.hpp key on BND_MATH_NO_FP. Every dbl:: reference already sat in an #else branch, so the integer engine is selected cleanly.

Proof (not just "it compiles")

New single_header_nofp_smoke target + Linux CI step compiles the single header with -DBND_MATH_NO_FP and a poison <cmath> shim first on the include path — the build hard-errors if any <cmath> is pulled in. Confirmed locally that it fires without the macro (genuine) and passes with it.

Verification

• default engine 397/397, CORDIC engine 440/440
• single_header_nofp_smoke builds & runs FP-free; single_header_smoke / single_header_freestanding_smoke still build
• amalgamation regenerated; amalgamate_up_to_date green

🤖 Generated with Claude Code