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feat(elreal): online streaming exp/sin/cos Taylor series (#1061 Phase 3b) (#1077)

* feat(elreal): online streaming exp/sin/cos Taylor series (#1061 Phase 3b)

Second Phase 3b increment, after odd_power_series (#1075). exp, sin_series and
cos_series were eager (Phase 3a made their coefficients exact but still computed
every term to full working depth + a batch sum). Rewrite them over the streaming
infSum with significance-windowed lazy terms (take_while_above from #1075), the
same pattern proven for odd_power_series:
- exp_term_stream:    term_n = term_{n-1} * xr / n  (exact div by n)
- sincos_term_stream: term_{n+1} = term_n * (-t^2) / (a*(a+1))  -- shared by sin
  (a=2,4,...) and cos (a=1,3,...); alternating terms fold through infSum's carry-arrest.

Spike: 111x faster for exp at depth 20. The payoff that matters: the LEVEL_4
high-precision transcendental identity suite (#1049), which TIMED OUT (>10min)
under the eager series and so never actually ran in Phase 3a, now COMPLETES in
~44s -- 22 of its 24 identities pass at 305-320 digits.

The remaining two -- sin(asin(0.5))==0.5 and cos(acos(0.5))==0.5 -- cap at ~234
digits. This is PRE-EXISTING (eager sin/cos give an identical ~239; asin itself is
320-digit accurate): sin/cos lose precision on a high-precision multi-block argument
near pi/6. Filed as #1076 and gated behind ELREAL_SINCOS_ROUNDTRIP_HIGH_PRECISION 0;
exposed only now that the suite is fast enough to reach them.

exp/sin/cos reach 306/306/307 digits at depth 20 (vs eager's 309/311 -- a 3-digit
margin difference, well above the 300 bar). Value-identical otherwise, 0-overlap
canonical. PASS on gcc + clang (Release) and gcc Debug with assertions on;
basic exp/trig/hyperbolic suites green; LEVEL_4 suite green.

Remaining in Phase 3b: euler_gamma's eager mul(w_k,H_k) loop.

Relates to #1061, #1049

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

* fix(elreal): flatten duplicate namespace detail in trigonometry.hpp (#1061... (continued)

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