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docs/pre13-low-c2.md

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+# Pre13 L-C2
+
+The two planned zero-length on-disk tail arrays now use standard flexible array
+declarators:
+
+- `erofs_xattr_ibody_header::h_shared_xattrs`
+- `erofs_xattr_long_prefix::infix`
+
+The layout probe retains the previous `[0]` declarations and asserts equal
+structure size/alignment and member offsets. Host Clang accepts both old and
+new declarations with the same warning/error result and identical layouts.
+The probe is also retained for the final FreeBSD guest compiler gate.
+
+No KLD build, QEMU, smoke, or feature test was run for this batch.