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feat(elreal): full-precision exp/sin/cos coefficients (#1061 Phase 3a, closes #1058) (#1074)

exp, sin_series and cos_series scaled each Taylor term by a HOST-DOUBLE
reciprocal coefficient -- mul_scalar(1.0/n, ...) and 1.0/denom -- which capped
the whole series at host-double precision (~17 digits) no matter the depth
(#1058). Same bug class as the e_zbcl constant (#1054), but in the FUNCTIONS.

Divide by the integer n (and (2n)(2n+1) / (2n-1)(2n)) EXACTLY instead, via a
single-block div() (from_native(n) is exact for n < 2^53). With the online
single-block division now fast (#1069), this removes the cap with NO default-
path regression:

  exp(1):   17 -> 309 digits at depth 20
  sin(0.5): 17 -> 309 digits
  cos(0.5): 18 -> 311 digits
  default depth=4 timing unchanged (exp 0.002s, sin 0.39s, before == after)

High-precision identities now hold to the working precision and scale with
depth (178-196 digits agreement at depth 10, exact 320 for asin+acos==pi/2).

Re-enable the #1049 gated checks in transcendentals_highprecision.cpp:
- ELREAL_EXP_SERIES_HIGH_PRECISION (this fix), and
- ELREAL_ADD_EXACT_CANCEL_FIXED (the #1057 exact-cancel add, merged via #1064).

This is Phase 3a of #1061 (the correctness half: remove the host-double cap).
Phase 3b -- making the series genuinely pull-driven/memoized over online
mul/div, which would also make the LEVEL_4 depth-20 identity suite fast rather
than eager-slow -- remains.

Basic transcendental tests PASS on gcc + clang; gated LEVEL_4 path compiles on
both. The LEVEL_4 high-precision identity suite is correct but eager-slow at
depth 20 (the motivation for Phase 3b); it runs in the stress tier only.

Closes #1058
Relates to #1049, #1061

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

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