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>

tests/js/tuning_display.test.js

@@ -9,7 +9,6 @@ const vm = require('node:vm');
 const APP_JS = path.join(__dirname, '..', '..', 'static', 'app.js');
 const HIGHWAY_JS = path.join(__dirname, '..', '..', 'static', 'highway.js');
 const V3_HTML = path.join(__dirname, '..', '..', 'static', 'v3', 'index.html');
-const V2_HTML = path.join(__dirname, '..', '..', 'static', 'index.html');
 
 function extractBlock(src, startMarker) {
     const start = src.indexOf(startMarker);
@@ -97,11 +96,6 @@ test('V3 index.html defines hud-tuning', () => {
     assert.match(html, /id="hud-tuning"/);
 });
 
-test('V2 index.html defines hud-tuning', () => {
-    const html = fs.readFileSync(V2_HTML, 'utf8');
-    assert.match(html, /id="hud-tuning"/);
-});
-
 test('highway.js updates hud-tuning from song_info tuning offsets', () => {
     const src = fs.readFileSync(HIGHWAY_JS, 'utf8');
     assert.match(src, /getElementById\('hud-tuning'\)/);