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>

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: