elreal Phase 7: transcendental functions and numbers

[Ravenwater]

Parent epic: #923
Depends on: Phase 6 (multiplication / division)

Goal

Provide the math suite: `exp`, `log`, `sin`, `cos`, `tan`, `sqrt`, `atan`, `asin`, `acos`, `sinh`, `cosh`, `tanh`, `pow`, `hypot`. Plus the named constants: `pi`, `e`, `ln(2)`, `log2(10)`, `euler_gamma`, `phi`, `sqrt(2)`, `sqrt(3)`, `sqrt(5)`.

Algorithm choices (per dissertation 4.2.7)

• Square root: Newton-Raphson on `ZBCL`
• exp: Taylor series, with argument reduction `exp(x) = exp(x/2^k)^(2^k)`
• log: Taylor series of `ln(1+u)` with argument reduction `log(2^k * m) = k*ln(2) + log(m)`
• sin / cos: Taylor series with Payne-Hanek-style range reduction modulo 2*pi (using `pi` itself as a ZBCL)
• atan: Machin formula or its extensions for fast pi convergence
• pi: BBP digit extraction, Machin, or AGM (the dissertation discusses options)
• e: Taylor series of `exp(1)` -- direct
• ln(2): Mercator series or related

Where exp_offset earns its keep

Transcendentals at extreme arguments (`exp(1000)`, `log(1e-300)`) have combined exponents far beyond any single host FP type's range. The `exp_offset` field on `block` lets a single block carry that exponent. Phase 7 will be the first phase where this matters in practice.

Acceptance criteria

• All listed functions implemented, each its own `.hpp` under `include/sw/universal/number/elreal/math/`
• All listed constants are lazy `ZBCL` factories (not eagerly materialised double constants)
• Tests check first 100 decimal digits of each constant against the 320-digit reference strings in `include/sw/math/constants/reference_constants.hpp` (PR feat(constants): high-precision (~320-digit) reference string constants #915)
• Function tests: roundtrip identities -- `log(exp(x)) == x`, `exp(log(x)) == x` for x > 0, `sin(asin(x)) == x` on `[-1,1]`, etc., stream-wise to depth N
• Pythagorean identity: `sin(x)^2 + cos(x)^2 == 1` stream-wise
• Tests parameterised over FpType in `{double, float, bfloat16}`
• Tests live in `elastic/elreal/math/`

Out of scope

• Gamma function, error function, special functions -- can be a follow-up
• BBP-style direct digit extraction for hex pi -- explicit non-goal (mentioned in epic non-goals)

Reference

McCleeary 2019 dissertation, Section 4.2.7 (transcendentals).