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docs/pre12-batch-09.md

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+# Pre-12 Batch 09: compression header responsibility
+
+Date: 2026-08-13
+
+## Scope
+
+- Removed the direct `internal.h` include from `decompressor.c` and the four
+  `decompressor_*.c` backend consumers that already include `compress.h`.
+- Made no declaration, definition, function, storage-duration, or public API
+  changes.
+
+## Candidate source
+
+Batch 04 created `compress.h` with a direct `internal.h` include and added
+`compress.h` to these five translation units while retaining their direct
+`internal.h` includes. Batch 06 then made LZMA, DEFLATE, and ZSTD backend
+functions private and exposed only their descriptor objects through
+`compress.h`. This left exactly five duplicate include edges attributable to
+Batches 04/06.
+
+## Static proof
+
+- `compress.h` remains the sole owner of `z_erofs_decompress_req`,
+  `z_erofs_decompressor`, the LZ4 decompressor prototype, and the three
+  backend descriptor declarations.
+- `internal.h` remains the sole public owner of `z_erofs_decompress` and
+  `z_erofs_parse_cfgs`; it does not include `compress.h`.
+- The include graph remains acyclic: each affected translation unit includes
+  `compress.h`, and `compress.h` includes `internal.h`, which includes only
+  `erofs_fs.h` among project headers.
+- Each affected consumer therefore still sees the same complete request,
+  descriptor, mount, map, and superblock types through one guarded include
+  path. No backend prototype or descriptor declaration is duplicated between
+  `internal.h` and `compress.h`.
+- The source diff removes exactly one `#include "internal.h"` line from each
+  of five authorized files and changes no other token.
+- `git diff --check` and `git diff --cached --check` were required for this
+  batch.
+
+## Deferred validation
+
+No build, QEMU, smoke, feature, or other dynamic validation was run under the
+execution constraint. The required FreeBSD KLD `WITH_ZSTDIO=0/1` matrix and
+final Plain/LZ4/LZMA validation remain deferred.