kaidokert / fixed-bigint-rs / 29715991077

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Constant-time shift-by-secret (ct_shl) + CT next_power_of_two for HeaplessBigInt (#172)

* Add ct_shl barrel shifter; make heapless Ct next_power_of_two constant-time

Reinstates the Ct NextPowerOfTwo dropped in #170, now genuinely CT.

ct_shl (shift.rs): a constant-time left shift by a SECRET amount. The
inherent Shl<usize> branches and cache-indexes on the shift amount, so
shifting by a secret leaks it (the #170 P1). The barrel shifter never lets
the secret drive control flow: each stage k shifts by the PUBLIC constant
2^k and applies-or-not via a masked ct_select on bit k of the amount;
over-width amounts mask to zero. The only ops touching the secret are `>> k
& 1` and a u32 `>=` — both branchless. O(width·log width), result width
self.len.

89 of 90 new or added lines in 3 files covered. (98.89%)

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