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docs/pre13-low-d.md

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+# 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.