Section-practice override 2/4 (core): activate and clear the practice-difficulty override from Section Practice #140

Effort: M

Part of the Section Practice difficulty override feature tracked in get-flashbacks/feedback-plugin-difficulty-ladder#160 (2/4). Builds on the Host capability shipped in #136 / PR #138 (static/capabilities/practice-difficulty.js). This is the producer integration: Section Practice (a core surface in this repo, static/js/section-practice.js) now drives the practice-difficulty override.

Note: "Section Practice" is not a standalone plugin. feedBack-plugin-riffrepeater and others treat core's Section Practice popover as a core surface (e.g. they call core's _sectionPracticeBarInnerHtml()). The producing repository is therefore this repo (feedBack core), not a plugin repository.

Scope

• Activate the practice-difficulty override for the looped section using the manual difficulty_pct pin, and clear it when the loop ends or Section Practice mode exits.
• Register/activate through the Host capability only — no direct writes to another plugin's state.
• The override is a manual percentage pin, not a second adaptive controller. Section Practice owns the practice workflow; Difficulty Ladder keeps owning song-wide adaptive difficulty.
• Track which loop the override belongs to so a foreign loop that clears or replaces ours clears the override exactly once (idempotent clear).

Integration points (current main)

• practiceSection(index, opts) — static/js/section-practice.js. Arm the loop via host.setLoop(start, end, ...) first, and only activate once the loop is confirmed, so a failed loop never leaves a live override.
• _setSectionPracticeMode(on, opts) — the single teardown funnel for turning the feature off, clearLoop(), and song/arrangement changes. Emit clear here when an override is active.
• _syncSectionPracticeFromLoop(...) — recognizes externally armed loops that match the current section and can turn Section Practice on. Decide whether a foreign matching loop should activate the core-owned override, or only a loop this feature armed.
• hasPhraseData() — sections with no authored phrases need an explicit answer, since the override currently changes nothing for them.
• Capability surface: static/capabilities/practice-difficulty.js (register-participant, unregister-participant, activate, clear; override-activated / override-cleared / override-rejected events).

Open decisions

1. Where does difficulty_pct come from? Core Section Practice has no difficulty control today, so the pin needs a source: a core UI, a participant registration from another plugin, or a persisted per-section value.
2. What is the stable core source ID? Unlike a plugin there is no manifest-based registration, so the capability call needs a fixed, documented source identifier (plus per-source cleanup for participant unregistration).
3. External loop ownership — activate on any matching external loop, or only on loops this feature armed?

Acceptance criteria

• With the practice-difficulty capability present, looping a section pins its effective difficulty and leaving the loop (or exiting Section Practice) restores the song-wide value.
• No activate is emitted when setLoop fails, and no override leaks after clearLoop(), song change, or arrangement change.
• Clearing is idempotent — a loop ended by another producer clears our override at most once.
• A matching issue/PR links back to feedback-plugin-difficulty-ladder#160 so the tracking issue can be unblocked.

Out of scope

Making Step Mode a difficulty provider (only if it explicitly opts in), per #22.