fix(nav): the library sometimes showed the legacy screen — map 'home' inside showScreen (#923)

Testers: "randomly, when moving to the library from another menu option, the library shows the
old interface — never when a song ends."

━━━ WHAT WAS ACTUALLY HAPPENING ━━━

#home is the PRE-V3 library screen. The v3 shell replaced it with #v3-songs, and the mapping DID
exist — but only inside WRAPPERS on window.showScreen, and only for callers that go through
`window`. THREE independent parties monkey-patch it, each capturing whatever happens to be there
at the time:

    app.js publishes the raw function
      -> shell.js wraps it, adding the home -> v3-songs mapping
      -> the stems plugin wraps it AGAIN (src/main.js:1029), capturing the current value

Plugins load ASYNCHRONOUSLY. The chain links up in whatever order the race settles, and any
capture taken before shell.js installs — or any re-assignment after it — silently drops the
mapping. Hence "randomly".

AND THE INTERNAL CALLERS NEVER TOUCHED window.showScreen AT ALL. closeCurrentSong and the
Esc-from-settings shortcut call the IMPORTED showScreen, which no wrapper ever sees. Reproduced
in a browser: the unwrapped function with 'home' lands on the dead legacy screen EVERY time.

"Never when a song ends" is the tell, and it is what identified the mechanism: closeCurrentSong
resolves its target through _resolvePlayerOrigin(), which ALREADY applies this mapping. That one
path was fine — which is exactly why the bug looked random rather than total.

PRE-EXISTING, not a regression from the module carve: the onclick="showScreen('home')" links and
the wrapper-only mapping both date to 2026-06-22.

━━━ THE FIX ━━━

The guard lives inside showScreen now: ONE place, in the function every caller routes through,
instead of a chain of monkey-patches that must each remember. Wrapper order stops mattering, and
the module-internal callers are covered for the first time.

Verified in a browser: the raw, unwrapped showScreen('home') — which reproduced as #home — now
lands on #v3-songs, and cannot be undone by any wrapper order.

━━━ AND A [P1] I INTRODUCED, WHICH CODEX CAUGHT ━━━

My first cut mapped BOTH 'home' and 'v3-home', copied straight from _resolvePlayerOrigin.

That is correct THERE and wrong HERE. _resolvePlayerOrigin computes where to RETURN TO after a
song, and landing on the Songs list from the dashboard is the right behaviour. But #v3-home is
the v3 DASHBOARD — a real screen that the shell's Home nav, the onboarding tour and the dashboard
re-render listener all target. Redirecting it would have made Home unreachable.

A LEGACY ALIAS IS NOT THE SAME THING AS A RETURN TARGET. Only 'home' is mapped now, and a test
pins that: re-adding 'v3-home' to the guard fails it.

4 tests, bite-tested both ways.

node 1049, pytest 2425, ESLint 0, Codex 0.

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

tests/js/show_screen_legacy_home.test.js

@@ -0,0 +1,85 @@
+// showScreen('home') must never land on the LEGACY library screen when v3 is present.
+//
+// Testers: "randomly, when moving to the library from another menu option, the library shows the
+// old interface — never when a song ends."
+//
+// #home is the pre-v3 library screen. The v3 shell replaced it with #v3-songs, and the mapping
+// DID exist — but only inside wrappers on `window.showScreen`, which fail two ways:
+//
+//   1. ORDER. THREE independent parties monkey-patch window.showScreen, each capturing whatever
+//      is there at the time: app.js publishes the raw function, shell.js wraps it to add the
+//      mapping, and the stems plugin wraps it again. Plugins load ASYNCHRONOUSLY, so the chain
+//      links up in whatever order the race settles. A capture taken before shell.js installs —
+//      or any re-assignment after it — silently drops the mapping. Hence "randomly".
+//
+//   2. THE INTERNAL CALLERS BYPASS window.showScreen ENTIRELY. closeCurrentSong and the
+//      Esc-from-settings shortcut call the IMPORTED showScreen, which no wrapper ever sees.
+//      Verified in a browser: the unwrapped function with 'home' lands on #home, always.
+//
+// "Never when a song ends" is the tell: closeCurrentSong resolves its target through
+// _resolvePlayerOrigin(), which already applied the mapping — so that one path was fine.
+//
+// The guard now lives inside showScreen itself: one place every caller routes through.
+
+const { test } = require('node:test');
+const assert = require('node:assert/strict');
+const fs = require('node:fs');
+const path = require('node:path');
+
+const SESSION_JS = path.join(__dirname, '..', '..', 'static', 'js', 'session.js');
+const src = () => fs.readFileSync(SESSION_JS, 'utf8');
+
+function bodyOf(name) {
+    const s = src();
+    const at = s.indexOf(`export async function ${name}(`);
+    assert.notEqual(at, -1, `${name} not found`);
+    let depth = 0;
+    for (let i = s.indexOf('{', at); i < s.length; i++) {
+        if (s[i] === '{') depth++;
+        else if (s[i] === '}' && --depth === 0) return s.slice(at, i + 1);
+    }
+    throw new Error('unbalanced');
+}
+
+test('showScreen maps the legacy #home library to #v3-songs', () => {
+    const fn = bodyOf('showScreen');
+    assert.match(
+        fn,
+        /id\s*===\s*'home'[\s\S]{0,80}getElementById\('v3-songs'\)[\s\S]{0,60}id\s*=\s*'v3-songs'/,
+        "showScreen must route 'home' to 'v3-songs' ITSELF — relying on a wrapper over "
+        + 'window.showScreen loses the mapping whenever a plugin wraps it first, and misses the '
+        + 'module-internal callers (closeCurrentSong, Esc-from-settings) altogether',
+    );
+});
+
+test('the guard runs BEFORE the screen is activated', () => {
+    const fn = bodyOf('showScreen');
+    const guard = fn.search(/id\s*=\s*'v3-songs'/);
+    const activate = fn.indexOf('classList.add(\'active\')');
+    assert.ok(guard !== -1 && activate !== -1);
+    assert.ok(guard < activate,
+        'the mapping must be applied before the screen is activated, or #home is shown first');
+});
+
+test('the guard is conditional on v3 actually being present', () => {
+    const fn = bodyOf('showScreen');
+    assert.match(fn, /getElementById\('v3-songs'\)/,
+        'the mapping must check #v3-songs exists — without it there is nowhere to route to');
+});
+
+test('it does NOT redirect v3-home — the dashboard is a real screen', () => {
+    // Codex [P1] on the first cut. _resolvePlayerOrigin() maps BOTH 'home' and 'v3-home' —
+    // correctly, because it computes where to RETURN TO after a song, and landing on the Songs
+    // list from the dashboard is right. Copying that condition into showScreen is NOT: #v3-home
+    // is the v3 DASHBOARD, which the shell's Home nav, the onboarding tour and the dashboard
+    // re-render listener all target. Redirecting it makes Home unreachable.
+    //
+    // A legacy alias is not the same thing as a return target.
+    const fn = bodyOf('showScreen');
+    // the condition, i.e. everything between `if (` and the `{` that opens `id = 'v3-songs'`
+    const m = fn.match(/if \(([\s\S]*?)\)\s*\{\s*id = 'v3-songs';/);
+    assert.ok(m, 'the legacy-home guard was not found');
+    assert.doesNotMatch(m[1], /v3-home/,
+        "showScreen must NOT redirect 'v3-home' — that is the dashboard, not the legacy library");
+    assert.match(m[1], /id === 'home'/, "it must still redirect the legacy 'home'");
+});