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fix(elreal): keep singleDiv's whole running remainder (#1561)

e_zbcl<half> stopped at 267 digits at every depth from 96 on. Single-block
division (singleDiv_step) truncated its running remainder, and its pending
quotient buffer, to kSingleDivCarryCap = 8 blocks. On half the remainder
reaches 13 blocks inside e_zbcl, so (1/9!) / 10 lost everything below
2^-883 and every later term inherited the error. On double it peaks at 4,
which is why only narrow hosts lost digits.

Both structures are exact sums and are now kept whole (singleDiv_whole);
kSingleDivCarryCap is removed. They stay O(1): each step consumes the
remainder's top block while the new residual lands at most about a block
below it. e_zbcl<half> reaches 320 digits from depth 96 on, at the same
cost.

BEHAVIOUR-CHANGE: elreal single-block division keeps every bit of its running remainder; e and other quotients on half are more accurate at depth

Resolves #1558

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

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