Pre13 L-D

The LZ4 two-byte little-endian offset now uses FreeBSD's unaligned-safe le16dec() helper. Boundary checks, input advancement, zero/back-reference validation, partial decoding, padding validation, and error returns are unchanged.

An exhaustive 65,536-value comparison proved ip[0] | (ip[1] << 8) and le16dec(ip) equivalent for every possible encoded offset. The final FreeBSD KLD and LZ4 smoke gates remain required later; this phase does not run them.