JOURNAL: 18g tidy follow-ups (stat-of-N + let-alias propagation)

Closes two of the three queued debts from the 18g tidy in the
same session. Tag-conditional partial-drop helper stays queued
— substantial, no observable fixture, all current fallback paths
cleanly use shallow ailang_rc_dec.

The 18-arc + 18g sub-arc + 18g tidy + post-tidy follow-ups are
all closed. Decision 9 has been re-framed to match the
orchestrator's dual-allocator stance (commit f10a77e). Next iter
is queue-driven; nothing load-bearing is open.

docs/JOURNAL.md

@@ -8279,3 +8279,75 @@ The three known debts from the 18g tidy (tag-conditional
 partial-drop, let-alias-aware mode propagation, bench-number
 stat-of-N) remain queued. They are RC-side improvements; none
 of them block the dual-allocator stance.
+
+## 2026-05-08 — 18g tidy follow-ups: stat-of-N + let-alias propagation
+
+Picked off the two smaller queued debts from the 18g tidy in
+the same session. The third (tag-conditional partial-drop
+helper) stays queued — substantively larger, no observable
+fixture, and the carve-outs it would close all fall back to
+`ailang_rc_dec` cleanly (potential leak, not crash).
+
+### Stat-of-N in the latency harness (commit `c2af5ad`)
+
+`bench/latency_harness.py` now accepts `--runs N`. Single-run
+output is byte-identical for back-compat. With N≥2 the report
+prints median + min..max per cell across runs; with N≥4 the
+slowest run is dropped before aggregation, matching
+`bench/run.sh`'s drop-slowest convention. `bench/run.sh`
+invokes the harness with `--runs 5` for each of the three
+latency arms.
+
+A 5-run smoke on the explicit-rc arm:
+
+```
+arm                       | median | p99(med)   p99(range) | p99/median(med)
+explicit @ rc (5-run)     |  226.1 |  296.4    [288.7, 311.3] |  1.31×
+```
+
+The qualitative claim ("RC tail latency is 23× better than
+Boehm; RC RSS is lower than Boehm") holds at this confidence
+level — variance is well below the signal. The exact JOURNAL
+2026-05-08 18g.2 numbers are now reproducible.
+
+### Let-alias-aware mode propagation (commit `2e00060`)
+
+Closes the carve-out shared by 18d.4 Iter A and 18g.1's
+pre-tail-call seam: `(let a t (match a ...))` where `t` is a
+non-Own fn-param defeated `scrutinee_is_owned`, because
+`current_param_modes` only registered fn-params and the
+let-binder `a` looked up as missing → default "owned" → arm-
+close drop fired on pattern-binders whose underlying memory
+the caller still owned.
+
+Fix shape: `Term::Let` lowering inspects `value`. If `value`
+is `Term::Var { name: src }` and `src` is in
+`current_param_modes`, the let-binder inherits that mode for
+the duration of the body. Push/pop, symmetric with `locals`.
+The Borrow-aliasing case is already prevented by the
+typechecker's `consume-while-borrowed` rule; the Implicit-mode
+case (default for unannotated params) is what the codegen
+carve-out actually surfaces, and it's what the new fixture
+`rc_let_alias_implicit_param.ailx` pins.
+
+Pre-fix on the new fixture: SIGSEGV / RC underflow on the
+second iteration of `loop`. Post-fix: matches `alloc=gc`
+(`0`). Kept as regression coverage. e2e count: 65 → 66.
+
+### Remaining queued debt
+
+- **Tag-conditional partial-drop helper.** Closes the two
+  dynamic-tag carve-outs (18d.4 match-arm shallow fallback
+  and 18g.2 App-binder partial-drop fallback). Substantial:
+  a per-type runtime helper that dispatches on the runtime
+  ctor tag and dec's non-moved ptr fields. No canonical
+  fixture currently hits the leak path; both fall back to
+  `ailang_rc_dec` (shallow) cleanly. Stays queued; will close
+  with the next iter that surfaces the leak in a benchmark or
+  a real workload.
+
+The 18-arc + the 18g sub-arc + the 18g tidy + the
+post-18g-tidy follow-ups: all closed. Decision 9 has been
+re-framed (commit `f10a77e`) to match the orchestrator's
+dual-allocator stance. Next iter is queue-driven; nothing
+load-bearing is open.