docs: 2026-09-29 session save -- the OOP arc (Clarion CLASS complete)

Consolidates v3-charcat-convresults, v3-class-lowering,
v3-virtual-dispatch, v3-class-finish and Showcase17/18/19: 147/147,
fixpoint OK (2,313,762 bytes). Tables each step's tag and commit;
summarises layout/THIS/calls, vtables/subtyping, sibling calls and
init bodies; records the one deliberate exclusion (multiple parents).

docs/summary_session-2026-09-29-oop.md

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+# Session save — 2026-09-29: the OOP arc (Clarion CLASS, complete)
+
+Suite **147/147**; self-hosting fixpoint **OK**
+(`bootstrap/fixpoint.sh`, image **2,313,762 bytes**). `master`, tree
+clean except the user's uncommitted `compiler/toto.mod`.
+
+This session took Clarion `CLASS` from "declared + checked only, ends
+in 230" to a complete single-inheritance object system with dynamic
+dispatch — in four tagged steps, each re-verified against the
+byte-identical fixpoint.
+
+## Steps, tags and commits
+
+| Step | Tag | Commit | What |
+| --- | --- | --- | --- |
+| CHAR-array `+` + ConvResults | `v3-charcat-convresults` | `5e2056f` | (warm-up: string `+` test + ISO `ConvResults`) |
+| Showcase 17 | `v3-showcase17` | `807ff9c` | CLASS + strings + conversions together |
+| CLASS lowering | `v3-class-lowering` | `ed02804` | fields, methods, `THIS`, single inheritance |
+| VIRTUAL dispatch | `v3-virtual-dispatch` | `98d3067` | vtables + subtype pointers |
+| Showcase 18 | `v3-showcase18` | `f584e62` | polymorphic Shape hierarchy |
+| CLASS finished | `v3-class-finish` | `3804256` | sibling calls + init body |
+| Showcase 19 | `v3-showcase19` | `9f1f278` | init body + sibling-call chain |
+
+## 1. CLASS lowering (`v3-class-lowering`)
+
+- **Layout** — a class is a record: `ComputeOffsets` gives
+  declaration-order offsets; single inheritance lays the child's
+  fields after the parent's, and `FindField`/`FieldOffset` walk the
+  parent chain.  `obj.field` and `WITH obj` reuse the record path.
+- **Receiver** — each method gets a hidden `THIS` `VAR` parameter,
+  pushed as the current `WITH` base, so a bare field name means
+  `THIS^.field`.
+- **Calls** — `obj.M(args)` binds statically; the receiver is the
+  hidden first argument; `CLASS IMPLEMENTATION` reuses the declared
+  method symbol (matching call-site and definition uids).
+- Docs: `docs/summary_class-lowering.md`.
+
+## 2. VIRTUAL dispatch (`v3-virtual-dispatch`)
+
+- **vtable** — each class with virtuals owns a slice of a shared
+  pool; the parent's slots are inherited first, a same-named virtual
+  method overrides in place, new ones append.
+- **vptr** — the introducing class gets an 8-byte vtable pointer
+  (offset 0, or after the parent's fields); subclasses share it, so
+  an object's vptr identifies its dynamic class.
+- **emission** — `data $vt_<typeindex> = { l $Method_uid, … }`;
+  a declared-but-unimplemented method is `l 0`.
+- **dispatch** — a virtual call loads the vptr, fetches the slot, and
+  calls indirectly; **subtyping** lets `POINTER TO Derived` assign to
+  `POINTER TO Base`.
+- Docs: `docs/summary_virtual-dispatch.md`.
+
+## 3. CLASS finished (`v3-class-finish`)
+
+- **Sibling-method calls** — inside a `CLASS IMPLEMENTATION`, a bare
+  method name is a call on `THIS` (chains work; a virtual sibling
+  still dispatches through the vtable).
+- **Class `BEGIN … END` init body** — emitted as `<Class>_init` and
+  run once at startup.
+- **Multiple parents** stay `230` (single inheritance by design).
+- Docs: `docs/summary_class-finish.md`.
+
+## Showcases
+
+- `Showcase17` — a `Bounded` counter class + `ConvResults` +
+  CHAR-array `+` → `showcase17 total=42`.
+- `Showcase18` — Circle/Square with a virtual `Area()`, driven through
+  an array of `POINTER TO Shape` → `showcase18 areas sum=139`.
+- `Showcase19` — an init body setting a global plus a sibling-call
+  chain → `showcase19 total=42`.
+
+## Tests (147)
+
+Added across the arc: `t_charcat`, `convresults_prog` (+ `conv_prog`/
+`showcase14` moved to `IntVal`/`RealVal`), `t_classmethod`,
+`t_classinherit`, `TClassDefProg` (`t_classdef.mod` →
+`t_classdef.def`), `t_virtual`, `t_classsibling`, `t_classinit`
+(`t_class` promoted from reject), and `Showcase17/18/19`.
+
+## Where CLASS stands
+
+✅ fields, methods (value/`VAR` formals, results), single inheritance,
+`WITH`, hidden `THIS`, `VIRTUAL` dynamic dispatch (vtables) with
+subtype pointers, sibling calls, init bodies.
+⏸ multiple parents only (deliberate).
+
+## Resume
+
+```sh
+cd .../m2compiler-V3
+./bootstrap/fixpoint.sh                       # FIXPOINT OK
+cd compiler && ./build.sh && ./run_tests.sh   # 147/147
+```
+
+## Remaining polish (beyond OOP)
+
+- `VAL(LONGINT|REAL, x)` and long→int narrowing.
+- More stdlib (`SysClock`, richer `Strings`/`Math`).
+- `Files`/`IOChan` extras (seek/pool nuances), deeper variant-record
+  support.