Initial commit: crosslang monorepo skeleton.

Umbrella repo gathering golang-lean (TGC), octive-lean (TOC),
tsm-lean (TSM), and common-lean (apex with cross-language
typeclasses). Subprojects arrive next via `git subtree add` so
per-file history survives the move.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

README.md

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+# crosslang
+
+Monorepo gathering language-implementation projects whose shared shape
+makes cross-language theorems possible. Each subdirectory remains a
+self-contained Lake package; `common-lean` is the apex that consumes
+the other three through Lake path-deps and exposes a typeclass-level
+abstraction over them.
+
+## Layout
+
+| Path | Role |
+| --- | --- |
+| `golang-lean/` | **TGC** — Go subset; big-step over `Heap × Env`. |
+| `octive-lean/` | **TOC** — GNU Octave; big-step over `Env`. |
+| `tsm-lean/`    | **TSM** — Tiny Stack Machine; small-step, positional operands. Also hosts a source-to-bytecode `Compile/` layer. |
+| `common-lean/` | Apex: `BigStepLang` and `SmallStepLang` typeclasses; TGC/TOC/TSM as instances; generic theorems that fire on every instance. |
+
+## Why a monorepo
+
+The three kernels share a `Core/` skeleton (`Syntax · Semantics · Types
+· Eval · Determinism · Preservation · TypeSoundness`). Anything *truly*
+language-invariant has to be statable in terms common to all three.
+Living together in one repo means:
+
+- a generic theorem proved in `common-lean` is a single edit away from
+  applying to every kernel,
+- new kernels are added by writing a `BigStepLang` (or `SmallStepLang`)
+  instance — no path-dep restructuring,
+- the factoring is post-hoc: abstractions land only after a third
+  concrete point demands them.
+
+## Build
+
+Each package builds independently with `lake build`. The apex:
+
+```sh
+cd common-lean && lake build
+```
+
+resolves path-deps to the three sibling kernels.
+
+## History
+
+Subprojects were imported with `git subtree add` so per-file `git log`
+continues to work across the move.