docs: session summary — constructors, corpus gaps, String/variant/nested

Covers v3-ctor-gaps, v3-gaps-1-5 and v3-string-variant-nested:
aggregate-constructor gaps, the nine corpus-driven features (libc,
enum/char sets, sized types, set builtins, forward types, opaque
String, variant-record assignment, nested modules, ADDRESS<->pointer),
and the corpus climb 283 -> 395 (testsuite) / 115 -> 131 (stdlib).
Suite 169/169; fixpoint OK (2,730,510 bytes).

docs/summary_string-variant-nested.md

@@ -0,0 +1,95 @@
+# Session summary — 2026-09-30 (b): constructors, corpus gaps, String/variant/nested
+
+Suite **169/169**; self-hosting fixpoint **OK** (image **2,730,510
+bytes**). `master`, tree clean except the user's uncommitted
+`compiler/toto.mod`.
+
+This session closed the aggregate-constructor work and then knocked out
+nine corpus-driven gaps.
+
+## Steps, tags and headline commits
+
+| Step | Tag | Commit | What |
+| --- | --- | --- | --- |
+| Constructor gaps | `v3-ctor-gaps` | `5a74f41` | set fields, string fold, index-type arrays, MIN/MAX |
+| Gaps 1–5 | `v3-gaps-1-5` | `619bff9` | libc, enum/char sets, sized types, set builtins, forward types |
+| This step | `v3-string-variant-nested` | `6882d63` | String, variant assignment, nested modules, ADDRESS↔pointer |
+| Summary | `v3-string-variant-nested-summary` | (this commit) | session save |
+
+(Showcase20, `v3-showcase20` / `b0494da`, opened the session.)
+
+## 1. Constructor gaps (`v3-ctor-gaps`)
+
+- **Set fields in record constructors** (`R{7, S{1,3,5}}`); the global
+  `braceCls` dispatch flag is saved/restored around a nested set
+  constructor.
+- **Constant string concatenation in constructor elements** —
+  `arraytype {Hello + " " + World}`: `CONST` string values,
+  `QbeGen.StrFold` folds `+` on constants, and a string element expands
+  into consecutive `CHAR` elements.
+- **Index-type arrays** `ARRAY <ordinalType> OF T`; enum values usable
+  as array indices.
+- **`MIN`/`MAX`** over ordinal types, arrays and sets.
+
+## 2. Gaps 1–5 (`v3-gaps-1-5`)
+
+1. **`FROM libc` intrinsic** — `printf`/`exit`/… import as permissive
+   external C procedures (varargs, no result); string/`CHAR`-array
+   actuals pass as `descriptor+8` (`SymTab.DeclareCProc`/`Varargs`,
+   `QbeGen.CArg`).
+2. **`SET OF enum` and `SET OF <char-range>`** (+ enum/`BOOLEAN`
+   subranges); set span limit 256 → 65536.
+3. **`MIN`/`MAX` for REAL/LONGREAL/LONGINT** and gm2 sized names
+   (`LONGCARD`, `SHORTREAL`, `INTEGER8/16/32/64`, `CARDINAL8/…`).
+4. **`INCL`/`EXCL`** statements and **`SHIFT`/`ROTATE`** expressions
+   (`QbeGen.SetClearBit`/`SetShift`, bit-by-bit over a runtime amount).
+5. **Module-level forward type references** — a global may name a type
+   declared further down; emission is deferred (`FlushPend`).
+
+## 3. String / variant / nested / ADDRESS (`v3-string-variant-nested`)
+
+- **Opaque/forward/undeclared aliases are pointer-like** — `ClassOf`
+  returns `ClPtr` for an unresolved alias, so `Design`/`DeclVar`/
+  `ResClass`/`AssOrCall` treat them as pointers; gm2's pervasive
+  **`String`** is predefined (imported definitions shadow it).
+- **Variant-record assignment** — `CopyRecord` is a flat byte blit of
+  `TypeSize(t)` (records are flat blobs; the only correct choice for
+  overlapping variant fields); variant records are emitted as one
+  zeroed blob.  Previously `q := r` compiled but corrupted data.
+- **Nested local `MODULE`** — verified `EXPORT` lists, procedures,
+  nested-nested modules and init bodies (already largely working).
+- **`ADDRESS` ↔ pointer assignment** — the ADDRESS checks now precede
+  the pointer-subclass test in `SymTab.Assignable` (`p := ADR(x)`).
+
+## Corpus (with `$V3_LIBS`)
+
+| milestone | testsuite | stdlib |
+| --- | --- | --- |
+| start of session | 283 | 115 |
+| `v3-ctor-gaps` | 301 | 115 |
+| `v3-gaps-1-5` | 376 | 124 |
+| `v3-string-variant-nested` | **395** | **131** |
+
+CRASH 0 throughout.  Remaining testsuite taxonomy: `undeclared
+identifier` 65, `not supported yet` 34, `invalid call` 27,
+`incompatible assignment` 12.
+
+## Tests (169, was 159)
+
+New: `t_ctor7` (set field + string fold), `t_minmax`, `t_minmax2`,
+`t_libc`, `t_setops`, `t_fwdtype`, `t_variant2`, `t_string`,
+`t_addrptr`, `t_nestedmod`.
+
+## Resume
+
+```sh
+cd .../m2compiler-V3
+./bootstrap/fixpoint.sh                       # FIXPOINT OK
+cd compiler && ./build.sh && ./run_tests.sh   # 169/169
+```
+
+## Remaining (see `docs/analysis_gaps.md`)
+
+`GOTO`/labels, result suffixes, generics, coroutines, multiple
+inheritance (deliberate), full forward *variables*, and the gm2-only
+library modules.