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feat(elreal): online streaming odd_power_series (#1061 Phase 3b) (#1075)

odd_power_series (artanh/atan: sum x^(2n+1)/(2n+1)) backs pi, ln2, ln10,
log2_10, log, atan, asin, acos. It was EAGER: compute every term to the full
working depth, accumulate in a vector, then a batch priestRenorm sum. For a
slowly-converging argument (artanh(1/3) for ln2 needs ~270 terms) that is the
elreal cost wall -- ln2_zbcl(12) measured 107s.

Rewrite it as the online streaming series the #1061 design calls for: generate
terms lazily (power_{n+1} = power_n * step via mul_online, term_n = power_n /
(2n+1) via div_online) and fold with the streaming infSum, each term and the
running power MATERIALISED ONLY TO ITS SIGNIFICANCE WINDOW (take_while_above):
a term whose leading exponent is already below the stop floor contributes
nothing, so the deep tail computes ZERO extra blocks. The windowing also breaks
the deep lazy mul/div nesting that would otherwise recurse per pull.

Measured (double host, spike + library):
  artanh(1/3) depth 20:   147x faster, value-identical
  ln2_zbcl(12):           107s  ->  ~1.1s
  pi/ln2/ln10 depth 20:   306 digits (unchanged, matches reference), ~1-1.8s
  el_math_constants test: ~98s -> 6.4s

Signature unchanged, so pi/ln2/ln10/log2_10/log/atan/asin/acos are untouched.
Value-identical and 0-overlap canonical (the heavily-overlapping terms fold
through infSum's carry-arrest). No default-path regression (faster at every
depth). PASS on gcc + clang (Release) and gcc Debug with assertions on;
constants / trigonometry / exponent / hyperbolic suites green.

This is the first increment of #1061 Phase 3b (genuinely pull-driven series over
online mul/div). The exp / sin / cos Taylor series (and euler_gamma's eager
mul(w_k,H_k) loop) remain eager -- follow-on increments.

Relates to #1061, #1049, #1053

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

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