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refactor(bigint): functional loop for leading-zero scan in from_string (#3638)

The leading-zero scan in `from_string_radix_pow2` used a `mut first`
threaded through a `while` loop and read out afterward. Carry it as a
functional `for first = start { ... continue first + 1 ... break first }`
loop instead, matching the loop style already used in this file
(e.g. the digit-accumulation loops). Behavior is unchanged: `first`
ends on the first significant digit (or `len` for an all-zeros input).

Add a regression test covering the scan's boundary cases across the
power-of-2 radix path: all-zeros (incl. negative-signed) normalizing to
0, leading zeros before a value, leading zeros spanning more than one
limb, and a single significant digit after a long zero run.

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

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