Adds training-subsystem observability to INFO compression and
integration tests exercising the full training + retirement lifecycle.

Metrics (additive to the last_duration_ms / last_sample_count fields
wired by #39):
  compression_training_state          idle/scanning/submitted/cooldown
  compression_training_scans_started  total scans initiated
  compression_training_successes      successful dict promotions
  compression_training_failures       aborts + ZSTD errors
  compression_training_cooldown_until_ms
  compression_training_current_db
  compression_training_dicts_retired  dicts that completed lifecycle (GC'd)
  compression_training_debug_frame_refs  active retiring dict frame_refs

Integration tests (tests/integration/compression-training.tcl):
  - First-training trigger fires and produces a usable dict
  - After training, new writes get compressed
  - Training does NOT fire below min keys
  - Training aborts + cooldown on ineligible values
  - Training scans across multiple databases
  - Dict retirement: compress 20 keys -> decompression mode ->
    read all -> dict GC'd (asserts frame_refs == 20, the regression
    guard for the double-count fix in #42)

The retirement test is FLUSHALL-free: 200 training keys are SET before
the dict exists (stay RAW, no refs); only 20 keys SET after the dict
is active get compressed.

Depends on #42 (install-path double-count fix), now merged to unstable.

Adds training-subsystem observability to INFO compression and
integration tests exercising the full training + retirement lifecycle.

Metrics (additive to the last_duration_ms / last_sample_count fields
wired by #39):
  compression_training_state          idle/scanning/submitted/cooldown
  compression_training_scans_started  total scans initiated
  compression_training_successes      successful dict promotions
  compression_training_failures       aborts + ZSTD errors
  compression_training_cooldown_until_ms
  compression_training_current_db
  compression_training_dicts_retired  dicts that completed lifecycle (GC'd)
  compression_training_debug_frame_refs  active retiring dict frame_refs

Integration tests (tests/integration/compression-training.tcl):
  - First-training trigger fires and produces a usable dict
  - After training, new writes get compressed
  - Training does NOT fire below min keys
  - Training aborts + cooldown on ineligible values
  - Training scans across multiple databases
  - Dict retirement: compress 20 keys -> decompression mode ->
    read all -> dict GC'd (asserts frame_refs == 20, the regression
    guard for the double-count fix in #42)

The retirement test is FLUSHALL-free: 200 training keys are SET before
the dict exists (stay RAW, no refs); only 20 keys SET after the dict
is active get compressed.

Depends on #42 (install-path double-count fix), now merged to unstable.

* feat(S1/S4): training lifecycle metrics + integration tests

Adds training-subsystem observability to INFO compression and
integration tests exercising the full training + retirement lifecycle.

Metrics (additive to the last_duration_ms / last_sample_count fields
wired by #39):
  compression_training_state          idle/scanning/submitted/cooldown
  compression_training_scans_started  total scans initiated
  compression_training_successes      successful dict promotions
  compression_training_failures       aborts + ZSTD errors
  compression_training_cooldown_until_ms
  compression_training_current_db
  compression_training_dicts_retired  dicts that completed lifecycle (GC'd)
  compression_training_debug_frame_refs  active retiring dict frame_refs

Integration tests (tests/integration/compression-training.tcl):
  - First-training trigger fires and produces a usable dict
  - After training, new writes get compressed
  - Training does NOT fire below min keys
  - Training aborts + cooldown on ineligible values
  - Training scans across multiple databases
  - Dict retirement: compress 20 keys -> decompression mode ->
    read all -> dict GC'd (asserts frame_refs == 20, the regression
    guard for the double-count fix in #42)

The retirement test is FLUSHALL-free: 200 training keys are SET before
the dict exists (stay RAW, no refs); only 20 keys SET after the dict
is active get compressed.

Depends on #42 (install-path double-count fix), now merged to unstable.

* test(compression): simplify first-training dict assertions

The first-training test had a confusing assertion:
  assert {$dict_after != $dict_before || $dict_before > 0}
The '|| dict_before > 0' clause made it vacuously true whenever a
dict already existed — the opposite of the intent. It was leftover
from shared-server (external) handling, but the test is external:skip
and always runs on a fresh instance where dict_before == 0.

Replaced with a direct fresh-instance check (active_dict_id == 0
before, > 0 after). Also dropped an unused dict_before capture in the
'does NOT fire' test (only known_dicts is asserted there).

* test(compression): reorder training tests so trigger gates are real

The old order trained a dict in test 1, leaving an active dict for the
rest of the shared-instance block. That made the negative-trigger tests
(below-min, ineligible-abort) pass vacuously: with a dict already
active, first-training can never fire regardless of key count or
eligibility (the trigger is gated on !active).

Reordered so all no-active-dict checks run first, then training fires
once:

  1. Below min keys -> no trigger (asserts scans_started == 0)
  2. Enough keys but none eligible -> scan fires + aborts
     (scans_started == 1, failures == 1) — metric-driven, not timing
  3. First-training across db0+db1 -> dict promoted (merges the old
     multi-DB test; only one first-training can fire per active dict)
  4. After training -> new writes compressed
  5. Active dict blocks a new first-training (new test for the
     'no retrain while a dict is active' invariant)

Test 2 arms the 30s cooldown; test 3 uses a 60s wait_for_trained_dict
to absorb it. Negative tests now assert on scans_started / failures /
successes counters instead of sleeping and hoping.