refactor(ui)!: remove the classic v2 shell — v3 is the only UI (R3a) (#871)

* refactor(ui)!: remove the classic v2 shell — v3 is the only UI (R3a)

Deletes `static/index.html`, the `/v2` route, and the `FEEDBACK_UI` v2/legacy
opt-out. `/` and `/v3` both serve `static/v3/index.html`, which has been the
default since 0.3.0.

This is step 0 of the core-frontend ES-module migration (R3a). Both shells load
the same `static/app.js`, so every later step of that migration — exposing the
window contract, the `defer` ordering fix, the `type="module"` flips — would
otherwise have to be made and verified twice. Removing the fallback now halves
that surface before any of it is touched.

Incidentally fixes a latent bug in `index()`: its guard read
`if getenv_compat("FEEDBACK_UI") or getenv_compat("FEEDBACK_UI") in ("v2", "legacy")`,
whose left operand is truthy for *any* non-empty value — so `FEEDBACK_UI=v3`
actually served the **v2** shell.

- `static/tailwind.min.css` regenerated: the content globs scanned the deleted
  file, so v2-only utility classes are now purged (CI's tailwind-fresh job
  rebuilds and diffs it).
- Constitution amended to 1.3.0 — Principle II's frontend file list now names
  `static/v3/index.html`.
- Tests: 4 suites read the v2 shell (3 via a constructed `path.join` that a
  literal grep misses). Their v2 halves are paired duplicates of v3 tests that
  stay, so they are dropped; `alpha_warning_banner` and the capability-registry
  script-order test retarget to `static/v3/index.html`.

BREAKING CHANGE: `FEEDBACK_UI=v2` / `=legacy` and the `/v2` route are gone.
Unset the variable and use `/`. No chart, settings, or plugin data changes, and
no plugin API changes — v3 reuses the same engine.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* docs: drop the stale '/ is v2' plugin-verification guidance (CodeRabbit)

The v3-only rewrite updated the intro paragraphs but left three lines that
still instructed plugin authors to verify in 'both / (v2) and /v3' — now the
same shell. Historical 'in v2 it was X' contrasts are kept: they still orient
authors whose plugins also ship to users on older cores.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -125,13 +125,13 @@ Notes:
 
 ### v3 UI (fee[dB]ack v0.3.0) — player-chrome contract
 
-v0.3.0 ships a redesigned UI behind a flag (`FEEDBACK_UI=v3` or the `/v3` route);
-the classic UI (v2) stays the default until 0.3.0 ships, so **plugins must work in
-both**. v3 reuses the same engine (`server.py`, `app.js`, `highway.js`, `playSong`,
+v0.3.0's redesigned UI is **the only UI** — the classic v2 shell and its
+`FEEDBACK_UI` / `/v2` opt-outs are gone, so there is no second shell to support.
+v3 reuses the same engine (`server.py`, `app.js`, `highway.js`, `playSong`,
 `showScreen`, capabilities, library providers, the `window.feedBackViz_<id>` /
 `setRenderer` contract), so a plugin's **backend, capabilities, `nav`/`screen`,
 visualization renderers, diagnostics, and settings export work unchanged** — v3
-surfaces `nav` in its sidebar and mounts screens exactly as v2 does.
+surfaces `nav` in its sidebar and mounts screens as before.
 
 **The only thing that changed is the player chrome.** If your plugin injects a
 control into it, you must adapt:
@@ -157,7 +157,7 @@ control into it, you must adapt:
   popovers 40).
 
 Full guide + the canonical snippet: **[docs/plugin-v3-ui.md](docs/plugin-v3-ui.md)**.
-Verify any player-injecting plugin in **both** `/` (v2) and `/v3`.
+Verify any player-injecting plugin at `/` — it and `/v3` serve the same v3 shell.
 
 ### Performance — never run DOM queries on a per-frame path
 
@@ -566,7 +566,7 @@ a local pointer + code map.
 - **Storage** — `localStorage` for all user preferences
 - **Styling** — Tailwind CSS utility classes, dark theme (`bg-dark-600`, `text-gray-300`, accent `#4080e0`, gold `#e8c040`). Tailwind is served as a **prebuilt** stylesheet (`static/tailwind.min.css`, regenerated by `bash scripts/build-tailwind.sh`), **never** the runtime Play CDN — the CDN's on-the-fly JIT rescanned the DOM on the main thread and dropped ~26% of frames with the 3D highway (feedBack-desktop#110). The committed CSS only contains classes the build scanner saw, so CI (`tailwind-fresh`) rebuilds and diffs it; run the build script and commit when you add new classes. A plugin that uses classes not guaranteed in core (notably arbitrary values like `w-[37px]`) MUST ship its own compiled stylesheet via the `styles` manifest key, built with `corePlugins.preflight = false` (utilities only — core ships the one base reset). Plugins MUST NOT load the Tailwind Play CDN or any runtime CSS JIT. See constitution Principle II.
 - **Naming** — camelCase for JS functions, kebab-case for CSS classes, snake_case for plugin IDs
-- **Text selection (v3)** — the v3 UI defaults to `user-select: none` on `html` (in `static/v3/v3.css`) so accidental drag/double-click selection of chrome never looks broken. Form fields are always re-enabled, and a **plugin's mounted screen subtree (`.screen[id^="plugin-"]`) stays selectable by default**, so a plugin's copy-worthy text (lyrics, chord names, results, diagnostics) is unaffected — *unless your plugin renders copyable content OUTSIDE its `plugin-<id>` screen* (e.g. injected into the player chrome / a HUD overlay), which inherits the non-select default. Opt such content back in with the core-served **`.fb-selectable`** class (it sets `user-select: text` on the element + descendants; works for runtime-installed plugins since it's hand-authored in core CSS, not a scanned Tailwind utility). Never use a `* { user-select: none }` rule (breaks input carets/IME), and never use `user-select: none` to "lock" text — keep errors, IDs, paths, versions, and metadata selectable. (v2 is unchanged.)
+- **Text selection (v3)** — the v3 UI defaults to `user-select: none` on `html` (in `static/v3/v3.css`) so accidental drag/double-click selection of chrome never looks broken. Form fields are always re-enabled, and a **plugin's mounted screen subtree (`.screen[id^="plugin-"]`) stays selectable by default**, so a plugin's copy-worthy text (lyrics, chord names, results, diagnostics) is unaffected — *unless your plugin renders copyable content OUTSIDE its `plugin-<id>` screen* (e.g. injected into the player chrome / a HUD overlay), which inherits the non-select default. Opt such content back in with the core-served **`.fb-selectable`** class (it sets `user-select: text` on the element + descendants; works for runtime-installed plugins since it's hand-authored in core CSS, not a scanned Tailwind utility). Never use a `* { user-select: none }` rule (breaks input carets/IME), and never use `user-select: none` to "lock" text — keep errors, IDs, paths, versions, and metadata selectable.
 - **Player layout** — `#player` is `display:flex; flex-direction:column; position:fixed; inset:0`. `#highway` is `flex:1`. `#player-controls` sits at the bottom. Hiding the highway collapses the layout — use `margin-top: auto` on controls if you need to hide it.
 
 ## Backend Conventions