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feat(elreal): named constants as lazy ZBCL factories (Phase 7.1, #931) (#1039)

* feat(elreal): named constants as lazy ZBCL factories (Phase 7.1, #931)

Generated (not baked-in) constant factories on the Phase 5/6/7.2 foundation:
- pi      via Machin: 16*atan(1/5) - 4*atan(1/239)
- e       via the exp(1) Taylor series (sum 1/n!)
- ln2     via 2*artanh(1/3); ln10 via 3*ln2 + 2*artanh(1/9)
- log2(10)= ln10 / ln2
- sqrt2/3/5 via Newton-Raphson sqrt (Phase 7.2); phi = (1+sqrt5)/2
- euler_gamma: host-precision seed (no efficient elementary series -- the one
  exception; a high-precision expansion is a follow-up)

detail::odd_power_series implements the shared atan/artanh odd-power series,
clamped to stop a margin above the denormal floor.

Host scope: the series constants (pi/e/ln2/ln10/log2(10)) are tested on
{double, float} -- bfloat16's 8-bit exponent range cannot hold the deep
expansions these series generate. The radicals (sqrt2/3/5, phi) and euler_gamma
stay in range and are tested on {double, float, bfloat16}. On a double host pi
agrees with a 3-limb high-precision reference to > 100 bits (precision well
beyond the host double); full 100-digit reference-string verification is a
follow-up (needs a decimal->dyadic parse).

e_zbcl bounds its factorial loop at FpType's smallest normal so it never feeds a
subnormal term to from_native.

All 28 elreal tests pass on gcc and clang (Release) and with assertions on;
RISC-V cross-compiles clean; ASCII clean.

Relates to #931 (sub-phase 7.1).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* feat(elreal): include constants.hpp in the umbrella header (#931 7.1)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* docs(elreal): correct bfloat16 series-constant rationale (mantissa, not exponent)

The header comment wrongly blamed bfloat16's exponent range; bfloat16 has the
SAME 8-bit exponent range as float. The real reason series constants are scoped
to {double,float} i... (continued)

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