[PATCH] Fix ChaCha SIMD feed-forward carry (GH #1362, PR #1363)

[PATCH] Fix ChaCha SIMD feed-forward carry (GH #1362, PR #1363)

The NEON, SSE2 and Altivec backends applied the per-block counter offset with a 64-bit add after the rounds. A carry from word 12 could therefore alter word 13 of the keystream.

Precompute each block's initial state, then use 32-bit feed-forward adds. Add regression vectors covering ChaCha8/12/20 and counter offsets 1, 2 and 3.

This matches the earlier AVX2 fix in GH #1069.

Gbp-Pq: Name fix_ChaCha_SIMD_feed-forward_carry.patch