The COW-invariant test enables compression (master=compression,
sweeper, an imported dict) and leaves compressed values in the
keyspace. In --external shared-server mode this pollutes the server
for every subsequent test file. The next file, unit/other, runs
DEBUG DIGEST, whose computeDatasetDigest -> xorObjectDigest ->
mixStringObjectDigest path iterates the kvstore directly and calls
getDecodedObject() on the leftover compressed value, hitting
serverPanic("Unknown encoding type") (object.c) -> server crash ->
the external CI jobs (test-external-{standalone,cluster,nodebug})
fail with "I/O error reading reply".

Add an end-of-file cleanup test that flushes the keyspace (dropping
all compressed frames) and turns the feature off, leaving the shared
server clean. dict-max-versions is left at 16 (not reset to the
default 4) so later suites that import dicts into the shared registry
have headroom.

This is test hygiene only. The underlying DEBUG DIGEST crash on
compressed values is a real, pre-existing product bug (a kvstore-
direct reader that does not decompress, the same class PR #24 fixed
for rdbSaveStringObject / AOF rewrite but missed for the digest
path). It is tracked separately and must be fixed before transparency
mode (full corpus under --compression) lands.

* test(compression): COW-invariant merge-blocker (S2.13)

Add tests/unit/compression-cow-invariant.tcl, the runtime form of the
R2.4.5 COW-audit checklist and a v1 merge blocker per design §7.2.

The feature relies on two correctness invariants when a value is, or is
being, compressed:

  (1) Decompress-before-mutate. lookupKey(...,LOOKUP_WRITE) permanently
      decompresses any OBJ_ENCODING_COMPRESSED value to RAW before the
      command handler runs, so no in-place byte-mutating command ever
      operates on a compressed frame (which would corrupt bytes or hit
      getDecodedObject()'s serverPanic — COMPRESSED is neither
      sdsEncodedObject nor INT).

  (2) Worker-snapshot immutability (R2.4.4). While a worker reads a
      value's sds bytes, the value is pinned (refcount>=2), so a
      concurrent in-place mutation COWs via dbUnshareStringValue,
      leaving the worker's bytes untouched.

Both are code-discipline, not type-enforced. The test exercises them at
runtime across every in-place string mutator (APPEND, SETRANGE, SETBIT,
BITFIELD SET, GETSET, GETDEL, SET-overwrite), a 200-iteration mutation
storm against the live worker pool, and a transient-view + write-path
interaction. Each asserts the result matches the value semantics
computed independently in Tcl, that compression_errors_total stays 0,
and that the mutated value re-compresses and round-trips.

The audit behind the test found no violators: t_string.c, bitops.c,
hyperloglog.c, module.c (DMA write / truncate / OpenKey), and debug.c
all honor the write-lookup decompress + dbUnshareStringValue discipline.

Auto-discovered via the tests/unit/*.tcl glob; skips cleanly under
BUILD_ZSTD=no (no gen-zstd-dict helper). Compression is reached via
COMPRESSION SWEEP FORCE (the deterministic operator trigger) rather than
async enqueue timing; startup raises compression-dict-max-versions and
flushes for --external shared-server headroom.

* test(compression): clean up shared-server state in COW-invariant test

The COW-invariant test enables compression (master=compression,
sweeper, an imported dict) and leaves compressed values in the
keyspace. In --external shared-server mode this pollutes the server
for every subsequent test file. The next file, unit/other, runs
DEBUG DIGEST, whose computeDatasetDigest -> xorObjectDigest ->
mixStringObjectDigest path iterates the kvstore directly and calls
getDecodedObject() on the leftover compressed value, hitting
serverPanic("Unknown encoding type") (object.c) -> server crash ->
the external CI jobs (test-external-{standalone,cluster,nodebug})
fail with "I/O error reading reply".

Add an end-of-file cleanup test that flushes the keyspace (dropping
all compressed frames) and turns the feature off, leaving the shared
server clean. dict-max-versions is left at 16 (not reset to the
default 4) so later suites that import dicts into the shared registry
have headroom.

This is test hygiene only. The underlying DEBUG DIGEST crash on
compressed values is a real, pre-existing product bug (a kvstore-
direct reader that does not decompress, the same class PR #24 fixed
for rdbSaveStringObject / AOF rewrite but missed for the digest
path). It is tracked separately and must be fixed before transparency
mode (full corpus under --compression) lands.

* test(compression): tag COW-invariant test external:skip

The previous cleanup-at-end approach did not fully solve the shared
external-server pollution: this test churns global compression state
and cannot restore a pristine registry (a dict that ever held frames
is not reliably reclaimable today, so it lingers). That broke
unit/type/compression's documented-defaults / clean-registry
assertions, which run after this file in --external mode.

Tag the file external:skip instead. The test deliberately reconfigures
the server globally (master switch, sweeper, imported dict, compressed
frames), which makes it a poor citizen on a shared, externally-managed
server. Its COW-correctness value is fully exercised in normal
(dedicated-server) mode, the primary CI mode, where the server is torn
down between files. This matches the precedent of other stateful tests
(dump, expire, introspection, ...).

Removes the now-unneeded end-of-file cleanup test.

* docs(plan): mark S2.13 (COW audit) + S6.1 (cow-invariant test) done

PR #41 delivers the S2.13 COW audit (no violators) and the S6.1
merge-blocker test tests/unit/compression-cow-invariant.tcl. Record the
audit outcome, the external:skip rationale, and the separately-tracked
DEBUG DIGEST / getDecodedObject crash discovered during the audit
(Phase-B blocker, follow-up PR). Update the Phase-A override note to
reflect S4.x (#39) and S2.13 (#41) as done.

* test(compression): fix stale comments after external:skip change

The startup and cow_configure comments still described running 'against
the same server as the integration suite in --external mode'. With the
file now tagged external:skip that's inaccurate — it runs only against a
fresh dedicated server. Reword to reflect that the startup flush + cap
headroom are defensive clean-slate setup, not shared-server handling.

* fix(compression): decode compressed values in DEBUG DIGEST readers

DEBUG DIGEST and DEBUG DIGEST-VALUE iterate the kvstore directly
(computeDatasetDigest via kvstoreIteratorNext; DIGEST-VALUE via dbFind)
so a debug command can digest logically-expired keys. They therefore
bypass the lookupKey() transient-view decompression hook and observe raw
OBJ_ENCODING_COMPRESSED robjs.

Both funnel a string value through xorObjectDigest's OBJ_STRING branch
into mixStringObjectDigest / xorStringObjectDigest, which called
getDecodedObject() — that panics on any encoding other than
RAW/EMBSTR/INT (serverPanic 'Unknown encoding type'), crashing the
server on the first compressed value.

Route the two digest helpers through objectGetUncompressedView (R2.5.2),
the single decoder primitive PR #24 used for rdbSaveStringObject /
rioWriteBulkObject. For a compressed value the helper decompresses into
a scratch sds (read directly — not via getDecodedObject, since
incrRefCount on the OBJ_STATIC_REFCOUNT view would panic); otherwise it
falls back to getDecodedObject (still handles INT). No-op passthrough
for non-compressed values and for BUILD_ZSTD=no.

The digest is thus defined over logical (decompressed) bytes: identical
whether a value is stored compressed or not. Pre-existing crash on
unstable; a blocker for transparency mode (full Tcl corpus under
--compression, which runs DEBUG DIGEST extensively).

Test tests/unit/compression-debug-readers.tcl (external:skip): compressed
values -> DEBUG DIGEST / DIGEST-VALUE don't crash, digest is byte-identical
compressed vs decompressed, and a DEBUG RELOAD round-trip preserves both
digest and values.

* docs(compression): record S2.14 (DEBUG DIGEST reader fix)

Mark S2.14 done in plan.md (the follow-up to the S2.13 audit finding) and
list DEBUG DIGEST / DIGEST-VALUE among R2.5.7's explicit-decompression
kvstore-direct readers in the design doc.