Draw extension decorations, and fire the active-editor event when the editor exists

[Seungpyo1007]

Track C of the 0.3.0 plan, item 2. Closes the last place in the shim that answered successfully and did nothing.

Decorations

createTextEditorDecorationType returned a working-looking handle; setDecorations accepted ranges. Nothing was ever drawn.

The two APIs have different shapes — VS Code takes CSS directly, Monaco takes class names and expects those classes to exist somewhere. So each type compiles to a rule in a stylesheet and Monaco gets the name.

• src/ext/decorations.ts holds the conversion as pure functions, tested directly. It also escapes quotes and backslashes in an extension's contentText; without that a crafted string breaks out of the rule and takes the whole stylesheet with it.
• One shared stylesheet, not one <style> per type — extensions that colour per line create and drop types constantly, and per-type elements would grow head into the thousands.
• An empty array means "clear what this type drew", and unloading an extension collects what it drew.
• Creating a type whose options yield nothing drawable logs a warning instead of quietly succeeding — that being the shape this whole area exists to remove.

The bigger find underneath

onDidChangeActiveTextEditor was handing extensions the wrong editor.

openFile fired the notification before its setState, so no pane and no model existed yet — and notifyExtensions bailed at if (!doc) return before reaching the active-editor fire. Opening a file therefore delivered nothing, and the one event that did arrive carried the previous file's editor.

Extensions that draw on editor changes — which is most of the ones that decorate — were drawing onto whatever was open before. The event now waits for the pane to actually exist (bounded, 1s) and fires exactly once per file, from either the open path or the pane mount.

Verification

A temporary extension was installed and driven through CDP. 7/7: the rule reaches the stylesheet, after content included; the classes land in the editor DOM; the computed background is rgba(143, 168, 147, 0.35) and the computed ::after is " ← SZDECO_OK" — actual pixels, not just markup; switching tabs redraws (event counter confirms two events); an empty array clears to zero.

1009 unit tests passing (28 new). npm run typecheck and npm run build clean.

A note on how this was verified

Three probe runs during this work tested a stale bundle. The Vite dev server had died, and the app falls back to dist/ silently, so changes made after the last build were not in what the probe exercised — which is what made this bug look unfixable for several rounds. The runs reported above are from a fresh build.