Replace per-job refcount model with QSBR (Quiescent-State-Based
Reclamation) for dictionary lifetime management between main thread
and compression workers.

Key changes:
- §4.4: full QSBR model description with struct updates
  (retire_worker_gen[], atomic active pointer, retiring list, GC)
- §4.4: comparison table of both approaches with rationale
- §4.6: compressionJob no longer carries CDict*; worker loads active
  dict atomically at compress time; reports quiescent after use
- R2.11.4: updated worker contract (load, use, report quiescent)
- §3: architecture bullet updated to match

Rationale: QSBR avoids passing lifetime-managed pointers across thread
boundaries — a pattern that memory-safe languages (Rust) prohibit
without Arc<T> and that in C relies on manual refcount discipline
across all control paths. The QSBR model concentrates complexity in
the registry and gives workers a minimal, hard-to-misuse contract.

Addresses 5 of 6 review comments on the QSBR design. Comment #6
(`compressionJob.key` extra-lookup concern) is explicitly deferred to
a follow-up PR per reviewer guidance.

Comment #1 (line 428) and #5 (line 544) — drop language-comparison
framing:
  Removed all references to Rust / `Arc<T>` / "memory-safe languages"
  / `shared_ptr` from §4.4 intro, the "Why QSBR" bullet list, and the
  §4.6 "Why the worker loads the active dict itself" paragraph. The
  rationale now stands on its own technical merit (decoupling the
  registry from worker hot paths; minimal worker contract;
  safe-directional failure modes) rather than via comparison to
  another language's type system. C with explicit protocols is the
  right tool for this problem; the comparison added rhetorical
  weight without adding signal.

Comment #2 (line 326) — duplication with R2.11.4:
  §3.3 Separation invariants restated the worker contract that
  R2.11.4 already specifies authoritatively. Slimmed the §3.3 bullet
  to a one-liner that points at §2.11 R2.11.4 and §4.4. Eliminates
  drift risk between the two places.

Comment #3 (line 439) — bound the retiring list, block on cap:
  Added new step 7 to the QSBR section explaining the cap interaction
  with R2.3.3. The retiring list is a subset of `dicts[]`, capped at
  `compression-dict-max-versions`. When grace-barrier draining cannot
  keep up (worker starvation, persistent `frame_refs > 0`), the cap
  is reached and BOTH training AND promotion are refused per R2.3.3:
  `LL_WARNING` log entry, `compression_dict_cap_reached` set in INFO,
  operator intervention required (raise cap or run COMPRESSION SWEEP).

Comment #4 (line 449) — grace-barrier wake-up via cond_broadcast:
  The original step 6 proposed enqueueing barrier jobs into the SPMC
  inbox to force idle workers to advance generations. This doesn't
  actually work: under work-stealing semantics a single fast worker
  can drain all barrier jobs while siblings stay asleep on the cond
  var. Rewrote step 6 to use a wake-all primitive built on
  `pthread_cond_broadcast`, and added a "Wake-all primitive"
  paragraph to §4.6 that describes extending `mutexqueue.h` with two
  new APIs: a broadcast wake-all (for QSBR grace barriers, config
  changes, etc.) and a shutdown-signal variant (for pool teardown).
  Step 6 cross-references §4.6 for the mechanism.

Comment #6 (line 513) — DEFERRED:
  Reviewer flagged that `compressionJob.key` (a `robj *` carried in
  the job) implies the main thread does an additional lookup at
  install time, doubling the per-write lookup cost. The reviewer
  explicitly tagged this as "follow up PR" — addressing it would
  require a redesign of the install-side data flow and is out of
  scope for the QSBR design change. Tracked as an open item; will
  be addressed before code lands for the install path (S2.7 in
  the implementation plan).