avnd: support comboboxes with runtime-populated item lists

[bltzr]

PR (ossia/score): runtime-populated comboboxes for avnd objects

Branch: feature/dynamic-combobox (off master)
Depends on: avendish feature/dynamic-combobox (companion PR).
Diff: 4 files, +119/-10.

What

Lets an avendish object repopulate a combobox port's item list at run time —
e.g. a "File" combobox that shows the audio files currently in a folder,
updating as the folder changes. Consumes the avendish halp::dynamic_combobox
port.

Changes

• Process::ComboBox (WidgetInlets.hpp/.cpp): setAlternatives() +
  alternativesChanged signal. setAlternatives refreshes the value domain
  and re-clamps the current value when the list shrinks. Alternatives were
  already serialized, so documents reopen with their last-known items.
• Widgets (ControlWidgets.hpp): the inspector and canvas comboboxes read
  their items live and rebuild on alternativesChanged (previously the list
  was fixed at widget construction).
• Executor (Crousti/Executor.hpp): connect_dynamic_items injects the
  port's update_items callback, marshalling runtime updates to the model
  inlet on the main thread (QPointer-guarded). Nodes with type-only inputs
  are skipped.

Compatibility

Purely additive. Objects and ports that don't use the feature are unaffected;
the widget/executor paths are guarded by requires/if constexpr on the new
members.

Testing

A throwaway DynComboTest object (in the score-avnd-flucoma addon) lists a
folder's audio files in a dynamic_combobox and follows folder changes live;
verified in the running app. Not part of this PR.

[bltzr]

Depends on the avendish port: celtera/avendish#171 (halp::dynamic_combobox). That should land first — this PR consumes its update_items callback.

[bltzr]

Closing for now — the avendish port this consumes (celtera/avendish#171) has no real consumer yet, so parking both until one exists rather than adding to the review queue. Kept on bltzr:feature/dynamic-combobox; will reopen together with the avendish PR when adopted.

[bltzr]

Reference, not an active review request — parked alongside the avendish port it consumes (celtera/avendish#171). Working example: DynComboTest.

We solved our own case (Granola's sound picker) with a custom widget rather than this binding, so nothing depends on it. Happy to revive both together if you think a runtime-populated combobox is worth having generally.

[jcelerier]

eventually it's a feature we want though ! it's very useful in many cases

[bltzr]

OK then I'll reopen it and move this forward, as it's going to be useful for the flucoma port

[bltzr]

Runtime-tested end-to-end now (populate at document load, save/reload round-trip, selection preserved). Two additions since last push:

• ComboBox now serializes its folder-port metadata, so a loaded document re-scans the folder and keeps live refresh (previously factory-set only, lost on deserialization).
• QGraphicsCombo: clicking a combobox on the canvas now opens a QMenu popup (items = current alternatives). Before, the only interaction was the drag-to-spin, which made comboboxes effectively read-only on the process panel while working in the inspector — this fixes that for every combobox/enum port, independently of this feature.

celtera/avendish#171 should still land first.

[bltzr]

Follow-ups, all runtime-tested:

• Folder-backed comboboxes now install a QFileSystemWatcher, so files written while the document is open (an upstream analysis, a recorder…) appear in the list immediately.
• Value refresh keeps a selection that names a not-yet-existing file (it becomes selected once written) instead of clamping to an arbitrary entry.
• The sibling port may also hold a file path — the file's containing folder is listed then, so a picker can sit next to an existing sound-file port without a dedicated folder port.

Exercised across ~45 processes in a FluCoMa addon: full analysis chain (onset slicing → corpus building → KD-tree fit/query) runs zero-config with the pickers.