feat(thenable): wasm-path thenable-resolution primitives (Closes #205)

The wasm/Node backend cannot await a host Thenable, so a
languageClientSendRequest result was unconsumable by a wasm guest
(blocking the #103 rsr-certifier pilot data path). The #199
function-value ABI now makes the host→guest re-entry expressible, so
the minimal #103 design sketch is finally buildable.

- stdlib/Vscode.affine: `thenableThen(t, on_settle: fn(Unit) -> Int)
  -> Disposable / Async` and `thenableResultJson(t) -> String`.
- packages/affine-vscode/mod.js: thenableThen wraps the guest closure
  via the PR-5c closure-pointer marshalling (wrapHandler) and stores
  the settled value keyed by the Thenable handle; thenableResultJson
  returns it JSON-encoded via the established reg(string) convention
  used by every other `-> String` extern. Per-process __thenableResults
  map. Rejection path stored as { __error }.

Verified: Vscode.affine typechecks; dune test --force 253/253, zero
regression; mod.js node --check clean; a consumer using both primitives
typechecks and lowers (`thenableThen(t, ((u) => 0))`).

Unblocks the rsr-certifier pilot (PR-5d-B): sendRequest rsr/getCompliance
→ thenableThen/thenableResultJson → status-bar/diagnostics/webview.

Closes #205
Refs #103, #199

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

Author: hyperpolymath

packages/affine-vscode/mod.js

@@ -33,6 +33,8 @@ module.exports = function makeVscodeBindings(vscode, lcModule, hostShim) {
   const reg = (obj) => hostShim._registerHandle(obj);
   const get = (h) => hostShim._getHandle(h);
   const getInstance = () => hostShim._instance;
+  // Settled host-Thenable values, keyed by Thenable handle (issue #205).
+  const __thenableResults = new Map();
 
   // ── String marshalling ─────────────────────────────────────────────
   // AffineScript's WASM 1.0 codegen stores string literals at the offset
@@ -333,6 +335,34 @@ module.exports = function makeVscodeBindings(vscode, lcModule, hostShim) {
       const ctx = get(ctxHandle);
       return reg(ctx ? ctx.asAbsolutePath(readString(relPtr)) : "");
     },
+
+    // ── Thenable resolution (issue #205) ───────────────────────────
+    // The wasm guest cannot await; these let it observe a settled host
+    // Thenable. thenableThen registers the guest closure (reusing the
+    // #199 closure-pointer marshalling via wrapHandler) and stores the
+    // settled value keyed by the Thenable handle; thenableResultJson
+    // returns it JSON-encoded (same reg(string) return convention as
+    // every other `-> String` extern).
+    thenableThen: (tHandle, onSettlePtr) => {
+      const thenable = get(tHandle);
+      const cb = wrapHandler(onSettlePtr);
+      if (!thenable || typeof thenable.then !== "function") {
+        return reg({ dispose() {} });
+      }
+      Promise.resolve(thenable).then(
+        (val) => { __thenableResults.set(tHandle, val); try { cb(); } catch (_e) {} },
+        (err) => {
+          __thenableResults.set(tHandle, { __error: String(err) });
+          try { cb(); } catch (_e) {}
+        }
+      );
+      return reg({ dispose() {} });
+    },
+    thenableResultJson: (tHandle) => {
+      if (!__thenableResults.has(tHandle)) return reg("");
+      try { return reg(JSON.stringify(__thenableResults.get(tHandle))); }
+      catch (_e) { return reg(""); }
+    },
   };
 
   const VscodeLanguageClient = {

stdlib/Vscode.affine

@@ -305,3 +305,20 @@ pub extern fn extensionAbsolutePath(ctx: ExtensionContext, rel_path: String) ->
 /// explicit `Unit` param avoids the zero-param-fn return-type collapse).
 /// Returns the progress Thenable.
 pub extern fn withProgressNotification(title: String, work: fn(Unit) -> Thenable) -> Thenable / Async;
+
+// ── Thenable resolution (issue #205) ─────────────────────────────────
+//
+// The source-to-source backend can `await` a Thenable directly; the
+// wasm/Node backend cannot, so these primitives let a wasm guest
+// observe a settled host Thenable. `thenableThen` registers a guest
+// callback (the #199 function-value ABI — the host re-enters it when
+// the Thenable settles); `thenableResultJson` reads the settled payload
+// JSON-encoded once the callback has fired.
+
+/// Run `on_settle` when `t` resolves. `on_settle` is a function value
+/// (#199); the host invokes it after settlement. Returns a Disposable.
+pub extern fn thenableThen(t: Thenable, on_settle: fn(Unit) -> Int) -> Disposable / Async;
+
+/// The resolved value of `t`, JSON-encoded. Empty string until `t` has
+/// settled (i.e. until the `thenableThen` callback has fired).
+pub extern fn thenableResultJson(t: Thenable) -> String;