Implements the declarative master-switch model from PR-B (design doc):
replaces the bool `compression-enabled` config with the 3-state enum
`compression-master-switch` (compression / decompression / off, default
off), adds the `compression-active-sweeper` enum (enabled / disabled,
default disabled) and `compression-active-sweeper-interval` time config
(default 0 = no periodic re-runs). Matching INFO fields. Apply hooks
implementing the R2.1.5 transition rules and the R2.1.6 non-functional
state warning. C1 stops at config plumbing; the transient-view drain
mode dispatch is C2, the sweeper engine + COMPRESSION SWEEP FORCE
command are C3.

Master-switch transition table (R2.1.5):
  compression -> decompression : auto-retire active dict (drain mode)
  compression -> off           : NO retire (state frozen)
  off         -> decompression : auto-retire active dict if any
  decompression -> off         : no action (already retired)
  off -> compression           : no action; existing dict reusable
  decompression -> compression : no action; falls into R2.1.7 third state

Apply-hook implementation notes:

- The hooks track the prior value via file-static caches (one for
  master, one for active-sweeper, one shared for the warning detector
  + a parallel for compression-threads). compressionInit() calls
  syncApplyHookCaches() AFTER boot config is applied, which fixes two
  bugs:
    1. Boot-time non-functional warning (master=compression+threads=0
       set at boot) didn't fire because apply hooks don't run during
       boot config load.
    2. An operator booting with master=compression and later setting
       master=decompression would have seen the apply hook log a
       misleading "off -> decompression" transition (because the
       static was initialized to the default, not the boot value).

- applyCompressionThreads in config.c calls compressionAfterThreadsApplied
  to keep the warning detector's threads cache in sync.

- maybeWarnNonFunctional emits LL_WARNING once on transition INTO
  master=compression+threads=0; subsequent CONFIG SETs that don't
  leave the state are no-ops.

Stale-reference sweep (PR-B vocabulary update):

- src/compression.c: 4 comments and 2 production checks updated from
  "compression-enabled" / server.compression_enabled to master-switch
  language. INFO field rendering switched from compression_enabled:0
  to compression_master_switch:<name> + new compression_active_sweeper
  + compression_active_sweeper_interval fields.
- src/compression_workers.c: 1 comment line.
- src/compression_train.c: gated cron entry on master==compression
  instead of !enabled bool.
- src/compression_registry.h: 2 comments referencing
  "COMPRESSION SWEEP direction=decompress" updated to the new operator
  surface (master=decompression + active-sweeper=enabled).
- src/compression_registry.c: log message at dict-cap-reached.
- src/compression.h: removed unused compressionToggle() declaration
  (was a Phase-0 stub, never called); replaced with the apply-hook
  declarations.

Test updates:

- src/unit/test_compression_eligibility.cpp: replaced
  server.compression_enabled with server.compression_master_switch
  using the new enum constants. Added a new test case
  MasterSwitchDecompressionBlocksEligibility verifying that the
  predicate also rejects in master=decompression (drain mode).
- src/unit/test_compression_train.cpp: same conversion.
- src/unit/test_compression_workers.cpp: 1 comment update.
- tests/unit/type/compression.tcl: rewrote 5 tests that referenced
  compression-enabled to use compression-master-switch. Added two
  new tests verifying the enum accepts each documented value plus
  rejects out-of-set values for both master-switch and active-sweeper.
  Updated the 18-knob defaults check (was 16 knobs; the rename from
  1 bool to 1 enum + 2 new sweeper configs increases primary-knob
  count from 5 to 6, advanced unchanged at 11, hence 17... wait,
  the test count is 18 because compression-active-sweeper-interval
  is a separate config registration. Confirmed by the test passing.)

Verified:

  make -j2 -C src SERVER_CFLAGS=-Werror   # clean (BUILD_ZSTD=yes)
  make -C src distclean
  make -j2 -C src SERVER_CFLAGS=-Werror BUILD_ZSTD=no   # clean
  ./runtest --single unit/type/compression  # 13/13 pass

  Boot/runtime smoke:
   - --compression-master-switch compression --compression-threads 0
     emits boot-time LL_WARNING.
   - CONFIG SET master compression -> decompression -> off logs the
     transitions correctly with the right "from" values.
   - INFO compression and COMPRESSION STATUS render the new fields.

Not verified locally:

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

Three findings from a thorough re-sweep of the codebase for residual
PR-B-vocabulary references:

1. compressionCommand() still routed `enable` / `disable` subcommands
   to a Phase-0 stub (return OK silently), and the HELP text listed
   `ENABLE, DISABLE, SWEEP, TRAIN` as future Phase-1 work. Per
   PR-B's design §4.5, the ENABLE/DISABLE aliases are NOT part of
   v1 (the 3-state enum doesn't map cleanly to enable/disable
   verbs). Removed the stub branch and rewrote the HELP text to
   reflect the v1 surface (STATUS / HELP land here; SWEEP FORCE /
   TRAIN / DICT * land in subsequent S2 PRs).

2. Three code comments cited "R2.1.5" when describing the "no
   active dictionary" state. Per the post-PR-B design that's
   actually R2.1.7 (master=compression but no active dict yet —
   the third state). Updated:
     - src/compression.c:1158 (compressionEnqueueCandidate's
       no-dict short-circuit)
     - src/compression.h:299 (header doc for the same)
     - src/compression_workers.c:619 (worker err>0 worker-policy
       sentinel comment)

3. .agents/planning/realtime-data-compression/idea-honing.md's
   PR-B header note said "Q1-Q16 hold up unchanged" but Q15
   (testing strategy — §7.1 transparency-mode harness) describes
   a `--compression` flag whose server-config payload uses the
   old `--compression-enabled yes` etc. The flag's design intent
   is still valid, but the specific config names are superseded.
   Tightened the header note to flag Q15 explicitly and revise
   the unchanged-Qs list (Q1-Q4, Q6-Q14, Q16).

Verified:

  make -j2 -C src SERVER_CFLAGS=-Werror   # clean (BUILD_ZSTD=yes)
  ./runtest --single unit/type/compression  # 13/13 pass