audit: cycle-close tidy for the measurement milestone (#280/#281/#282)

Architect drift review over 16ca4e1..HEAD: drift-clean but for one low
citation slip, now fixed here — snap_to_nearest_minute's doc cited the
record-then-replay boundary as C6 (firing policy); it is C11. What holds
(architect-confirmed): C27 taps match the contract (pure declaration,
compile-resolve+hoist through the shared lowering remap, caller-built
aura-core-only sink, unbound-is-inert, Tier-1 additive serde); the four #281
cells are C8/C6-clean (<=1 record/eval, warm-up None, When's quiet-None is the
Resample precedent, barrier stall correct); #280's snap respects C11 (node-local
reads only, firing/barrier timestamps untouched, stream verbatim); FlatGraph and
validate_wiring lockstep complete.

Regression (report-only bench): exit 0, all five surfaces fingerprint OK, deltas
within noise on a loaded box — no baseline update, nothing to ratify.

Scaling follow-up filed as #283: the single-run tap drain buffers O(cycles) rows
(the #138 profile) — fine for the bounded acceptance run, to be folded before a
full-history declared-tap study.

crates/aura-core/src/scalar.rs

@@ -23,7 +23,7 @@ impl Timestamp {
     /// early or late. Consumers deriving bucket membership or wall-clock
     /// minutes from `ctx.now()` snap the read here, once, rather than
     /// dividing/truncating the raw jittered stamp — the recorded stream
-    /// itself stays byte-verbatim (record-then-replay, C6); only the
+    /// itself stays byte-verbatim (record-then-replay, C11); only the
     /// consumer-side read is corrected.
     pub fn snap_to_nearest_minute(self) -> Timestamp {
         let m = Self::NS_PER_MINUTE;