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docs/pre12-batch-04.md

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+# Pre12 Batch 04: Private compression header
+
+## Change
+
+- Added `src/compress.h` for the decompressor request, descriptor, and backend
+  private declarations previously located in `src/internal.h`.
+- Added the header only to `decompressor.c` and the LZ4, LZMA, Deflate, and
+  ZSTD backend translation units.
+- Kept the runtime compression enum, `z_erofs_parse_cfgs`, and the public
+  `z_erofs_decompress` declaration in `src/internal.h`.
+- Preserved every migrated declaration signature and storage duration. No
+  function body, callback, conditional compilation block, errno path, or
+  buffer ownership changed.
+
+## Static validation
+
+- The `compress.h` consumer set is exactly the five planned decompressor
+  translation units; `super.c`, `zdata.c`, and all other public callers still
+  include only `internal.h`.
+- The include graph is acyclic: `compress.h` includes `internal.h`, while
+  `internal.h` does not include `compress.h`.
+- Exact searches found one definition of each migrated structure and one
+  declaration of each migrated backend function. The old declarations are
+  absent from `internal.h`.
+- The new header contains no Linux page, folio, bio, workqueue, XArray,
+  shrinker, compatibility, or related runtime declarations.
+- Source diff inspection and declaration normalization confirmed that this is
+  a declaration-only move plus the five include additions.
+- `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. The planned `WITH_ZSTDIO=0` and
+`WITH_ZSTDIO=1` KLD builds remain required during final validation.