docs: add instance-dispatch & birth invariants; smokes probe policy; archive CURRENT_TASK and replace with concise plan; impl VM stringify(Void) safety; tighten heavy probes; enable rewrite default ON

docs/design/instance-dispatch-and-birth.md

@@ -0,0 +1,34 @@
+# Instance Dispatch & Birth Invariants (Phase 15)
+
+Status: Adopt (Go). Scope: VM/Builder/Smokes. Date: 2025-09-27.
+
+## Goals
+- Unify user-instance method calls to functions for stability and debuggability.
+- Make constructor flow explicit: NewBox(Box) → Global("Box.birth/N").
+- Eliminate BoxCall-on-Void crashes (stringify, accessors) without changing prod semantics.
+
+## Decisions
+1) Instance→Function rewrite (default ON)
+- Builder always lowers `me.m(a,b)` to `Box.m/2(me,a,b)`.
+- Env override: `NYASH_BUILDER_REWRITE_INSTANCE=0|1` (default 1).
+
+2) VM BoxCall policy
+- User-defined InstanceBox: BoxCall is disallowed in prod; dev may fallback with one-line WARN.
+- Plugin/Builtin boxes: BoxCall allowed as before (ABI contract).
+
+3) NewBox→birth invariant
+- Builder: After `NewBox(Box)`, emit `Global("Box.birth/N")` when present (arity excludes `me`).
+- VM: No implicit birth; run what Builder emits.
+- Dev assert: `birth(me==Void)` forbidden; WARN+metric when hit.
+
+4) Void stringify safety valve (dev)
+- VM: `stringify(Void)` yields `"null"` (to match `toString(Void)`); one-line WARN+metric.
+- Remove once hits converge to zero.
+
+## Smokes & Probes
+- Heavy JSON smokes use a probe that prints `ok`. Runner compares the last non-empty line exactly to `ok` (trimmed). Noise-safe, portable.
+
+## Acceptance
+- Quick: JSON apps green; user-instance BoxCall hits=0; stringify-void hits=0.
+- Heavy: nested/roundtrip PASS where parser is available.
+