fix(tuner): anchor injectPlayerButton to a direct-child button (feedBack#800)

`injectPlayerButton()` anchored the injected Tuner button with
`controls.querySelector('button:last-child')`, which can match a NESTED
button that is not a direct child of `#player-controls`. `insertBefore(btn,
nestedButton)` then throws `NotFoundError` (the reference node must be a
direct child); since injection runs from the tuner's `screen:changed`
handler, the throw propagated out of the player-screen transition and
stalled its render. The v3 path was already safe (plugin-control slot);
only the classic anchor was bad.

Use `:scope > button:last-of-type` (direct child only) with a
`parentNode === controls` guard before insertBefore, falling back to
appendChild. Bump plugins/tuner 1.3.3 → 1.3.4.

Test: tests/plugins/tuner/js/inject_player_button.test.js — extracts the
real function and runs it over a faithful DOM model whose insertBefore
enforces the direct-child invariant; the nested-last-button case reproduces
the throw on the old anchor and passes on the new one (5 tests).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: byrongamatos <xasiklas@gmail.com>

CHANGELOG.md

@@ -8,6 +8,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [Unreleased]
 
 ### Fixed
+- **Tuner: opening the player screen no longer throws `NotFoundError` and aborts the player render (feedBack#800).** `injectPlayerButton()` anchored the injected Tuner button with `controls.querySelector('button:last-child')`, which — unlike a `:scope`-scoped query — can match a **nested** button that is not a direct child of `#player-controls`. `controls.insertBefore(btn, nestedButton)` then throws `NotFoundError` (the reference node must be a direct child), and because the injection runs from the tuner's `screen:changed` → player handler, the throw propagated out of the player-screen transition and stalled its render (surfaced by a headless render of a notation arrangement; the v3 path was already safe via the plugin-control slot, only the classic path had the bad anchor). The anchor is now `:scope > button:last-of-type` (a direct child only) with a `parentNode === controls` guard before `insertBefore`, falling back to `appendChild`. `plugins/tuner` → 1.3.4. Tests: `tests/plugins/tuner/js/inject_player_button.test.js` (nested-last-button repro, direct-child insert, no-button append, idempotency, v3 slot path).
 - **Auto-sync: DTW step constraint — riff-based songs no longer produce garbage sync points.** `librosa.sequence.dtw`'s default step pattern allows unbounded horizontal/vertical path runs, and on music with long self-similar chroma stretches (riff-driven stoner/doom, drone sections) the flat cost surface let the warping path collapse — minutes of score mapped onto a single audio frame, so the per-bar warp imported charts wildly out of sync while reporting success (observed on a real 138 BPM tab: effective displayed tempo 159 BPM, three sync points sharing one audio timestamp). `_dtw_align` now uses the standard music-sync slope-constrained step pattern (`[[1,1],[1,2],[2,1]]`, local tempo ratio bounded to 0.5x–2x), which makes the degenerate path impossible, with a fallback to unconstrained steps when the global length ratio makes the constrained pattern infeasible (e.g. a tab aligned against a full-concert video). Validated on the failing song: coarse points track the recording 1:1, refined downbeats land on onset peaks at 3.3x background energy.
 
 ### Added