SELF-HOSTING FIXPOINT: V3 compiles itself to a byte-identical image

- QbeGen.InitHeap now initialises nested element sub-array descriptors (headers)
- self-compiled V3's gen_ssa/M2.ssa == gm2-built V3's, byte for byte
- suite 111/111

compiler/src/QbeGen.mod

@@ -2510,6 +2510,7 @@ PROCEDURE InitHeap (addr: ARRAY OF CHAR; t: SymTab.TypeIndex);
           NewTemp(ea);
           Op3L("add", ea, addr, nb);
           NewHeap(SymTab.ArrayElem(t), eb);
+          InitHeap(eb, SymTab.ArrayElem(t));  (* init the sub-array *)
           Revive;
           W("  storel "); W(eb); W(", "); WL(ea);
           INC(i)

docs/summary_fixpoint.md

@@ -0,0 +1,58 @@
+# V3 self-hosting fixpoint reached
+
+Tag `v3-fixpoint`. Main suite 111/111.
+
+## The milestone
+
+V3, **compiled by V3 itself**, compiles the whole compiler source set to
+a **byte-identical** `gen_ssa/M2.ssa`:
+
+```
+M2   (built by gm2 -fiso)  compiles  SymTab QbeGen M2S M2P M2(driver)
+                          ->  gen_ssa/M2.ssa         (1,991,439 bytes)
+M2self (built by V3 from that image)  compiles the same set
+                          ->  gen_ssa/M2.ssa
+cmp: *** IDENTICAL (FULL FIXPOINT) ***
+```
+
+So Stage 2 (V3 compiling itself) reproduces Stage 1's image exactly —
+the self-hosting fixpoint.
+
+## The last fix
+
+`QbeGen.InitHeap`, for a nested array (`ARRAY … OF ARRAY …`) on the
+heap/stack, allocated each element sub-array with `NewHeap` but never
+**initialised its descriptor header**. A local `ARRAY [0..15] OF Name`
+therefore had sub-descriptors with garbage headers, and passing
+`names[i]` to an `ARRAY OF CHAR` formal trapped in the scanner's
+`GetName`. Fix: call `InitHeap(eb, elem)` on each sub-array.
+
+Repro:
+
+```modula2
+TYPE Name = ARRAY [0..63] OF CHAR;
+PROCEDURE P;
+  VAR names: ARRAY [0..3] OF Name;  i: CARDINAL;
+  PROCEDURE Set (VAR s: ARRAY OF CHAR); BEGIN s[0] := 'x' END Set;
+BEGIN i := 0; Set(names[i]) END P;
+```
+
+## How we got here (this arc)
+
+From "sources compile and link" to the fixpoint, each step a real V3
+runtime/ABI bug with a minimal repro: nested modules; inline record
+array fields; the NUL-terminated string model; `p^` as a `VAR` actual;
+de-eagered NIL guards; off-by-one string scans; short-circuit `AND`/`OR`;
+and now nested-array heap initialisation.
+
+## Remaining (bootstrap polish)
+
+- `bootstrap/fixpoint.sh`: automate Stage1 gm2 → Stage2 V3 → Stage3 V3
+  and `cmp` the images (the manual chain above already shows Stage2 ==
+  Stage1). Stage-aware CR flag: `-m` for gm2, no `-m` for V3.
+- Cross-stage `FileIO` selection (stage 1 uses the host `FileIO`; stage 2+
+  the self-hosted `runtime/syslib/FileIO`).
+
+## Files
+
+`compiler/src/{QbeGen.mod,M2.lst}`.