perf(vscode): pre-spawned rs fmt standby for the active editor

.vscode/launch.json

@@ -12,13 +12,19 @@
       "runtimeExecutable": "${execPath}",
       "args": [
         "--extensionDevelopmentPath=${workspaceFolder}/packages/vscode",
-        "--disable-extensions",
-        // An isolated, auto-created profile, so personal user settings
-        // (formatters, format-on-save, keybindings) cannot leak into the
-        // playground. `--user-data-dir` would be the stronger isolation, but
-        // the extension-host debugger strips it; `--profile` is the supported
-        // mechanism for debug launches.
+        // An isolated, auto-created profile, so neither personal user settings
+        // (formatters, format-on-save, keybindings) nor personal extensions
+        // can leak into the playground — the profile starts with none
+        // installed, which is why `--disable-extensions` is absent: it would
+        // only add its "all installed extensions are temporarily disabled"
+        // notification to every launch. `--user-data-dir` would be the
+        // stronger isolation, but the extension-host debugger strips it;
+        // `--profile` is the supported mechanism for debug launches.
         "--profile=rstack-playground",
+        // Built-in extensions ignore the empty profile. Git is the one that
+        // speaks up: every fixture sits under this repo, so it offers to open
+        // the parent repository on each launch.
+        "--disable-extension=vscode.git",
         // A fresh profile would otherwise prompt for workspace trust and greet
         // with the welcome tour on every first launch.
         "--disable-workspace-trust",

CONTEXT.md

@@ -0,0 +1,19 @@
+# Ubiquitous language
+
+Glossary of terms used across rstack-editor. Code, docs, commit messages and reviews use these words with exactly these meanings.
+
+## Core
+
+- **Stack** — one tool integration (lint, test, fmt) hosted by the extension shell. A stack registers against the shell and reports status through it; stacks never own UI chrome.
+- **Shell** — the always-activating extension core: detection, status bar, output channels, settings migration, stack lifecycle.
+- **Detection** — the per-workspace-folder scan deciding which stacks a folder lights up. Detection signals are config files and installed tool binaries, never user settings.
+- **Gate** — the per-stack activation condition: detected, workspace trusted, and the enable settings on.
+
+## fmt
+
+- **Cold format** — a format request served by spawning a fresh `rs fmt` process at request time; the request pays the full process start-up cost.
+- **Standby** — the single pre-spawned `rs fmt` process held ready for one specific file, so the next format of that file skips the start-up cost. There is at most one standby, and it is only ever armed for the active editor's file ("the standby tracks the active editor"). An editor change that cannot be armed kills it; an editor holding nothing this stack formats leaves it to expire.
+- **Arm** — create the standby for a file. Arming happens when the active editor lands on an eligible file and again right after a format consumed the previous standby.
+- **Consume** — serve a format request with the armed standby. A standby serves exactly one request; a request the standby cannot serve falls back to a cold format.
+- **Hot format** — a format request served by consuming the standby.
+- **Expire** — kill an idle standby to reclaim its memory. An expired standby is not an error; the next eligible event simply arms a new one.

packages/vscode/AGENTS.md

@@ -31,7 +31,7 @@ One extension replacing the standalone `rstack.rslint` and `rstack.rstest` exten
 
 - The lint × `rstack.config.*` bridge was built and deliberately removed: a partial editor-side bridge gave wrong results, and a correct one needs upstream work first. `TODO(rstack-bridge)` markers carry the plan. Do not reintroduce a partial bridge.
 - The test × `rstack.config.*` bridge stays thin on purpose: it points the upstream machinery at rstack's shipped shim and lets the shim interpret the config inside the worker, same as the CLI. Never re-implement rstack config semantics in the extension.
-- The fmt stack is a spawn-per-request `rs fmt --stdin-filepath` MVP. Its cwd is the governing config directory because rs fmt resolves config from cwd only, and formatting errors are log-only by design. The endgame is an upstream LSP, so do not add a warm-process middle tier.
+- The fmt stack is a spawn-per-request `rs fmt --stdin-filepath` MVP. Its cwd is the governing config directory because rs fmt resolves config from cwd only, and formatting errors are log-only by design. A single pre-spawned standby that tracks the active editor (see CONTEXT.md) is the accepted, bounded exception to "no warm tier". Do not grow it into a daemon: no long-lived protocol, no process pool, no cross-request state. The endgame is an upstream LSP; the standby retires with it.
 - `projectModules.ts` has no cache-invalidation hook and restart must not grow one. Node's ESM registry is keyed by resolved URL and process-lifetime, so clearing the local memo hands back the identical module object (verified); a `?epoch=` query does reload the entry but relative specifiers inside it do not inherit the query, yielding a fresh entry over stale dependencies. In-place reinstalls under an unchanged path need a window reload — say so, don't fake it.
 - The VSIX is platform-targeted for exactly one reason: the test stack's AST collection loads a native parser binding. Do not add another native dependency — it multiplies the release matrix.