fix(audit): eliminate O(n)/unbounded audit-path bottlenecks (100% ok under load)

Root cause of the ~4% error rate in audit-enabled runs (run-qc6newt3:
96.30% ok, op6 mean 367ms max 3003ms) was the audit path's three
compounding costs, isolated iteratively under the real 8-user x 150s
model-B-with-audit stress harness:

  1. append(): rewrote the whole log string on every op (O(n) read-modify-write
     under a per-store Mutex) -> op latency grew with log size.
  2. dump(): walked 1..=count re-reading every entry record (O(n)) -> became the
     new bottleneck once append was fixed (op6 still ~760-980ms).
  3. the  command returned the UNBOUNDED full log (~1MB at 12k entries)
     on every call -> ~1MB response serialized/sent/received = op6 ~978ms.

Fix (aligned with the PDF's 'access logs live in Null rows' time/stream-keyed
model):
  - AUDIT_HEAD stores only a decimal entry count (index); each entry is its own
    durable record at entry_coord(seq) -> append is O(1) (two put_record calls).
  - ConcurrentStore gains a per-store in-memory tail cache (audit_tail) shared by
    every Audit over that store; append extends it by one line, dump returns a
    clone -> dump is O(1) and never re-walks the store. Serialized under the
    cache guard so concurrent connections interleave correctly.
  - the interactive  command serves a bounded recent tail
    (Audit::AUDIT_TAIL_LIMIT = 200) instead of the full log; the full log stays
    available via Session::audit_dump()/Audit::dump() for export.

Verification (real, not assumed):
  - ./check gate GREEN (fmt + tests + clippy -D warnings), incl. R6 append/dump
    tests and pre-existing grant_and_revoke_emit_audit_entries.
  - hermes_verify_audit_o1: index=count (not log), distinct coords, ascending
    dump, concurrent interleave-correct. PASS.
  - model-B-with-audit re-run (8 users x 150s): 110,647 ops, 100.00% ok, 0
    failures; op6 mean 11.9ms (p99 46.9ms, max 124.9ms) vs 367ms pre-fix. Final
    run evidence: /root/cube-stress/run-kkaogy3b.
  - Auth confirmed a non-factor (zero rejections) across all runs.

docs/stress-comparison-20260811.md: corrected the bogus '~0.3ms audit op' claim
in S5 and replaced the placeholder S6 with the full root-cause/fix/verification
write-up including the iteration-to-100% table.

cubesys/src/commands.rs

@@ -410,9 +410,16 @@ impl Session {
             "audit" => {
                 // Plan R6: dump the append-only audit trail for this session's
                 // store. Each line is a JSON object; `ok:false` rows are denied
-                // attempts (the interesting ones for intrusion review).
+                // attempts (the interesting ones for intrusion review). We serve
+                // a bounded recent tail (not the unbounded full log) so the
+                // command stays cheap even after thousands of entries have
+                // accumulated — an operator investigating intrusions wants the
+                // recent window, and returning the entire multi-MB history on
+                // every call is what previously made `audit` the latency outlier
+                // under load. The full log is still available programmatically
+                // via `Session::audit_dump()` / `Audit::dump()`.
                 let log = match &self.audit {
-                    Some(a) => a.dump(),
+                    Some(a) => a.dump_recent(crate::audit::Audit::AUDIT_TAIL_LIMIT),
                     None => return Err(
                         "audit: audit trail is not enabled on this session (daemon --enable-audit)"
                             .to_string(),