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fix(elreal): exact-residual two_prod_host for odd-precision FpTypes (#942) (#1019)

* fix(elreal): exact-residual two_prod_host for odd-precision FpTypes (#942)

The generic Veltkamp/Dekker two_prod_host is exact only for even precision p:
the partial product a_hi*b_hi must fit in p bits (needs 2*ceil(p/2) <= p). For
odd p the split leaks error that grows with p.

Fix: keep Veltkamp/Dekker for even p (exact, stays in host arithmetic); for odd p
compute the residual a*b - p in a double intermediate (>= 2p bits for every host
FpType used in elreal blocks, p <= 26), so the residual is the exact value
rounded to the host type. Only the residual uses the wider type; the rounded
product p = a*b still uses host arithmetic.

Findings while implementing (the issue's premise was partly off):
- bfloat16 has digits=7 (odd) but its odd-p Veltkamp residual was already exact
  (0 ulp^2) -- the split error is negligible at small p. Unchanged.
- The real win is cfloat<24,5> (p=19): 79.6 -> 32 ulp^2.
- cfloat<>'s fma() does NOT help: it is not a fused op (rounds a*b before adding
  -p), so fma(a,b,-p) double-rounds (~32 ulp^2, no better than the double path).
  So issue Option 1 (FMA) is rejected in favor of Option 2 (wider intermediate).
- two_prod is bit-exact only when the residual is REPRESENTABLE in the host.
  cfloat<24,5> (es=5, p=19) underflows the residual into its subnormal range
  (~32 ulp^2 floor) -- a representability limit of the format, not an EFT defect;
  no Dekker/FMA/wider variant can recover it. Wide-exponent hosts (float, double,
  bfloat16) are exact (0 ulp^2).

two_mult.cpp: tightened the value-preservation tolerance 128 -> 64 ulp^2 to
reflect the post-fix bound (max observed ~32 for cfloat<24,5>); it cannot go to
~ulp^2 because of the cfloat<24,5> subnormal-residual floor.

Verified gcc+clang across two_sum/two_mult/two_div.

Resolves #942
Relates to #940 (elreal Phase 4 can now widen its FpType sweep)

Co-Authored-By: Claude Opus 4.7 (1M contex... (continued)

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