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docs/pre12-batch-05.md

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+# Pre12 Batch 05: ZSTD window lower bound
+
+## Change
+
+- Removed only the `rq->em->zstd_windowlog + 10 < 10` subcondition from
+  `z_erofs_zstd_decompress`.
+- Preserved the `> 20` upper bound, the `outputsize == 0` check, every
+  `return (-1)`, all ZSTD calls, resource release, and the complete disabled
+  `#else` stub.
+- Did not change ZSTD configuration parsing, accepted configuration range,
+  conditional compilation, errno behavior, or control flow after the guard.
+
+## Static proof
+
+- `struct erofs_mount.zstd_windowlog` is `uint8_t`, so integer promotion gives
+  a value in the range 0 through 255.
+- Adding 10 therefore produces a minimum value of 10; comparison with `< 10`
+  is unreachable for every representable field value.
+- `z_erofs_load_zstd_config` still rejects `windowlog > 10`, so configured
+  mount values and the remaining decompression upper-bound behavior are
+  unchanged.
+- Normalized function comparison confirms that the only removed logical
+  tokens are the single unreachable lower-bound subcondition and its `||`.
+- `git diff --check` and `git diff --cached --check` completed without errors.
+
+## Tests not run
+
+Per the execution constraints, no build, QEMU run, smoke test, feature test,
+or other dynamic test was run. A FreeBSD KLD build with `WITH_ZSTDIO=1`
+remains mandatory during final validation so this enabled function body is
+compiled.