…ess (writes) (2/3)

PR 2 of 3 in the S2.8 split. Wires the read-path transient-view model
from PR #21 (R2.5.7) + Appendix E.7 into lookupKey(), AND adds a
write-path permanent-decompress optimization with re-compression
auto-scheduling via signalModifiedKey().

== Read-path: transient view (R2.5.7 + Appendix E) ==

For lookupKeyRead* (no LOOKUP_WRITE), compressed values are decompressed
into a temp sds, the robj is registered in a per-server side-map and
pinned (incrRefCount); encoding flips to RAW for the duration of the
event-loop iteration. At the next compressionBeforeSleep() boundary,
the kvstore slot is re-fetched: pointer match → pointer-swap restore
(zero recompression cost), pointer mismatch → discard (mutation /
overwrite / expire / COW orphan). Memory bound is uncompressed-baseline
(at most the size the dataset would use without compression).

== Write-path: permanent decompress + signalModifiedKey enqueue ==

For lookupKeyWrite* (LOOKUP_WRITE set, no LOOKUP_NO_BYTES), compressed
values are PERMANENTLY decompressed in place: free the compressed
buffer, decRef the dict frame-ref, install fresh sds as val_ptr, flip
encoding to RAW. No side-map entry, no pin. Refcount stays 1.

Subsequent dbUnshareStringValue sees refcount==1 RAW → no COW → mutation
in place. Cheaper than transient view + COW, and avoids the wasted
beforeSleep kvstore re-fetch for a write that always discards.

The post-mutation value is re-enqueued for compression by hooking
signalModifiedKey(). signalModifiedKey is the canonical "logical value
at this key changed" signal — called by every byte-mutating command.
Audit confirmed: every relevant write path fires it; the 3 paths that
don't (RDB load per R2.6.2, worker drain per R2.9.2, module
SETKEY_NO_SIGNAL) are exactly the ones we want to skip.

R2.9.2 invariant intact: the direction is signalModifiedKey →
compression-enqueue, NOT the reverse. Compression infrastructure does
not call signalModifiedKey.

This replaces PR #19's compressionEnqueueCandidate calls in
dbAddInternal/dbSetValue, which fired too eagerly (worker-drain
re-enqueue → wasted predicate; empty initial-state values from
hashTypeLookupWriteOrCreate → wasted predicate) AND missed in-place
mutations like APPEND on a permanent-decompressed value (which never
go through dbReplaceValue).

== Implementation ==

src/server.h:
- Renamed LOOKUP_READ_BYTES → LOOKUP_NO_BYTES, semantic flipped to
  opt-out. Default behavior is "read bytes (decompress on demand)";
  metadata-only lookups (TYPE, EXISTS, OBJECT ENCODING, TTL, PERSIST,
  TOUCH, DEBUG POPULATE, OBJECT/MEMORY USAGE, setKey existence check)
  pass LOOKUP_NO_BYTES to opt out.

src/compression.{c,h}:
- Side-map: hashtable.c primitive keyed by robj pointer (default
  pointer-bits hash + pointer-equality compare); lazy allocation;
  early-out when empty.
- compressionMaterializeTransientView(): read-path decompress +
  register + pin + flip.
- compressionPermanentlyDecompress(): write-path decompress + free
  buffer + decRef dict + flip; no side-map, no pin.
- transientViewActive(): O(1) presence check for the deferred-capture
  fix (Appendix E.7).
- compressionBeforeSleep(): iterate side-map, restore via
  pointer-equality stale check, free per entry.
- compressionEnqueueModified(): looked up by signalModifiedKey;
  no-op when feature disabled / key deleted / value ineligible.
- compressionShutdown(): drain side-map (treat as discard).
- testOnlyCompressionDrainTransientViewAsDiscard(),
  testOnlyCompressionTransientViewSize(): test-only helpers.

src/db.c:
- lookupKey() splits decode strategy by LOOKUP_WRITE: write context
  → permanent decompress; read context → transient view.
- signalModifiedKey() now calls compressionEnqueueModified() as the
  third side effect (alongside touchWatchedKey + trackingInvalidateKey).
- Removed compressionEnqueueCandidate from dbAddInternal and
  dbSetValue (PR #19's wiring); replaced by the signalModifiedKey hook.
- typeCommand, existsCommand, setKey existence check pass
  LOOKUP_NO_BYTES.

src/object.c, src/expire.c, src/debug.c:
- objectCommandLookup, expire/ttl/persist/touch commands, DEBUG POPULATE
  pass LOOKUP_NO_BYTES.

src/networking.c:
- isCopyAvoidPreferred returns 0 when transientViewActive(obj) is true
  (deferred-capture fix per Appendix E.7).

src/unit/test_compression_transient_view.cpp (new):
- 9 gtest cases covering side-map lifecycle, materialize state machine,
  permanent decompress side-effects, decoder-error recovery.

== Verified locally ==

- make -j2 -C src SERVER_CFLAGS=-Werror clean.
- ./runtest --single unit/type/compression 10/10 pass.
- Manual smoke: server with --compression-enabled yes
  --compression-threads 2; PING/SET/GET/EXISTS/TYPE/OBJECT ENCODING/
  STRLEN/APPEND/TTL/TOUCH/DEL all work; clean shutdown.

== Not verified locally (CI gates) ==

- gtest unit tests (libgtest-dev not installed).
- clang-format-18.

== Diff stat ==

src/compression.c                            | 438 +++++++++++++++++++--
src/compression.h                            |  93 +++++
src/db.c                                     |  87 +++-
src/debug.c                                  |   5 +-
src/expire.c                                 |  16 +-
src/networking.c                             |  12 +
src/object.c                                 |   9 +-
src/server.h                                 |  26 +-
src/unit/test_compression_transient_view.cpp | 397 ++++++++++++++++++
9 files changed, 1037 insertions(+), 46 deletions(-)