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docs/pre13-low-a1.md

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+# Pre13 L-A1
+
+This batch aligns the remaining low-risk helper vocabulary with Linux:
+
+- `compress.h` now uses the Linux include guard name.
+- `erofs_inode_is_data_compressed()` replaces four equivalent FULL/COMPACT
+  tests that do not need to distinguish the two layouts.
+- `erofs_addrmask()` directly uses the existing 48-bit feature helper.
+
+Static proof:
+
+- For every value from zero through `EROFS_INODE_DATALAYOUT_MAX`, the new
+  compressed-layout helper equals the replaced boolean expression.
+- The helper was not substituted in `zmap.c`, where FULL and COMPACT remain
+  distinct cases.
+- `erofs_is_48bit()` and `erofs_sb_has_48bit()` tested the same incompat bit;
+  address-mask values and control flow are unchanged.
+- The old include guard and duplicate 48-bit helper have no remaining source
+  consumers.
+
+No build, QEMU, smoke, or feature test was run for this batch.