feat(stdlib): async-extern binding surface — Thenable + withProgress/sendRequest

#103 slice 1 (Promise-returning extern, per owner decision). The
AffineScript extern-call shape is synchronous, so Thenable-returning
vscode APIs could not be bound at all; the rsr-certifier vscode port
fell back to shelling out to the CLI.

Adds the binding surface only (smallest root-cause slice):
- stdlib/Vscode.affine: `extern type Thenable;` +
  `withProgressNotification(title, work_thunk) -> Thenable / Async`.
- stdlib/VscodeLanguageClient.affine:
  `languageClientSendRequest(c, method, params_json) -> Thenable / Async`.

No backend change needed: the Deno/Node source-to-source backend already
lowers an unlisted extern to a plain host call, so a Thenable-returning
extern emits a real JS Promise and a consumer's `await` is valid source
JS. The `Async` effect (v1 registry, PR-1 #196) tracks it in signatures.

Verified: both files typecheck standalone; dune test 253/253 (incl. the
AOT Vscode smoke); a consumer compiled with --deno-esm emits
`return languageClientSendRequest("rsr/getCompliance", "{}")` — a plain
awaitable host call.

Deferred to follow-up slices (per scope): vscode-host JS impl in
packages/affine-vscode/mod.js, and the rsr-certifier pilot restore.

Refs #103

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

Author: hyperpolymath

stdlib/Vscode.affine

@@ -283,3 +283,22 @@ pub extern fn processPlatform() -> String;
 /// `context.asAbsolutePath(rel_path)` — resolves a path relative to the
 /// extension's install location.
 pub extern fn extensionAbsolutePath(ctx: ExtensionContext, rel_path: String) -> String;
+
+// ── Async-extern ABI (issue #103) ────────────────────────────────────
+//
+// AffineScript's extern-call shape is synchronous, but most of vscode's
+// interactive surface returns a JS Thenable. `Thenable` is an opaque
+// host handle; the Deno/Node source-to-source backend lowers a
+// Thenable-returning extern to a plain host call, so the emitted JS is a
+// real Promise and a consumer's `await` is valid source JS. The `Async`
+// effect (v1 registry, #59) tracks this in signatures. The vscode-host
+// JS implementation lives in packages/affine-vscode/mod.js (follow-up
+// slice); these declarations are the binding surface + effect tracking.
+
+pub extern type Thenable;
+
+/// `vscode.window.withProgress({ location: Notification, title }, task)`.
+/// `work_thunk` is a wasm/host table index of the `() -> Thenable` task
+/// (same thunk-handle convention as `onDidSaveTextDocument`). Returns the
+/// progress Thenable.
+pub extern fn withProgressNotification(title: String, work_thunk: Int) -> Thenable / Async;

stdlib/VscodeLanguageClient.affine

@@ -31,3 +31,16 @@ pub extern fn newLanguageClient(id: String,
 
 pub extern fn languageClientStart(c: LanguageClient) -> Int;
 pub extern fn languageClientStop(c: LanguageClient) -> Int;
+
+// ── Async-extern ABI (issue #103) ────────────────────────────────────
+// Opaque host handle, declared per-module (these binding modules are
+// self-contained; the host erases the type). See stdlib/Vscode.affine.
+
+pub extern type Thenable;
+
+/// `LanguageClient.sendRequest(method, params)` — returns a Thenable of
+/// the JSON-encoded response. `params_json` is the request params encoded
+/// as a JSON string; the host shim parses it before the LSP call.
+pub extern fn languageClientSendRequest(c: LanguageClient,
+                                        method: String,
+                                        params_json: String) -> Thenable / Async;