Add dynamic_combobox: a combobox with a runtime-populated item list

[bltzr]

PR (avendish): dynamic_combobox — runtime-populated combobox port

Branch: feature/dynamic-combobox (off main)
Files: 2 new headers, 79 lines, no changes to existing code.

What

A combobox port whose item list is discovered by the processor at run time,
rather than fixed at compile time by range():

struct ins {
  halp::folder_port<"Folder"> folder;
  halp::dynamic_combobox<"File"> file;   // items filled at runtime
} inputs;

void operator()(int) {
  if (folder_changed && inputs.file.update_items)
    inputs.file.update_items(list_of_names);   // any thread
  // inputs.file.value is the selected index
}

• halp/dynamic_combobox.hpp — the port: int value, a static fallback
  range() (single "-" item), and
  std::function<void(std::vector<std::string>)> update_items.
• avnd/concepts/dynamic_items.hpp — avnd::dynamic_items_parameter<T>, so a
  binding can detect these ports.

Backwards compatibility

The design is additive and opt-in. Bindings that don't know the concept never
fill update_items; the port then renders as a plain combobox from the static
fallback range(). Existing objects and existing bindings are untouched — no
API, ABI or behaviour change.

Host side

The score binding that consumes this is a companion PR on ossia/score
(feature/dynamic-combobox): Process::ComboBox gains
setAlternatives() + alternativesChanged, the widgets rebuild live, and the
executor injects update_items (thread-marshalled). Other bindings can adopt
it incrementally or ignore it.

Motivation

Folder-backed pickers: the score-avnd-flucoma / Granola work needs a sound
whose choices are the files currently in a working folder. Also useful for OSC
address lists, dataset point ids, device names, preset names — any choice set
known only at run time. Full design notes in dynamic-combobox-proposal.md.

Open questions for review

1. Callback injection (chosen here) vs. reusing the processor→UI message bus.
2. Value semantics when the list changes: clamp index (host PR does this) vs.
   match by string.

Host binding

ossia/score#2110 consumes this port: Process::ComboBox gains a runtime setAlternatives() + alternativesChanged, the inspector/canvas widgets rebuild live, and the avnd executor injects update_items. This avendish PR should land first. No real object depends on it yet — it was verified with a throwaway test object.

[bltzr]

Host-side consumer: ossia/score#2110 (adds the Process::ComboBox runtime API + executor injection). This avendish PR is the standalone piece and can be reviewed independently.

[bltzr]

Closing for now — no real object consumes this port yet (Granola, the intended user, went with a custom picker), so I don't want to add to your review queue. The work stays on my fork branch bltzr:feature/dynamic-combobox; I'll reopen once a concrete object adopts halp::dynamic_combobox. Thanks!

[bltzr]

Leaving this as a reference rather than an active review request. A working example of the port in use: DynComboTest in score-avnd-flucoma — lists a folder's audio files in a halp::dynamic_combobox that repopulates live as the folder changes, verified in the running app (host binding: ossia/score#2110).

For our actual case — Granola's sound picker — we ended up doing it manually with a custom widget instead, so nothing here is blocking us. But if you think a runtime-populated combobox would be useful to others in the codebase, happy to revive and polish both PRs (this port + the score binding).

[bltzr]

related to ossia/score#2110 so reopening along with the latter

[bltzr]

Added an init file-name template parameter (folder_combobox<name, folder_port, extensions, init>): chains of processes that write then read conventionally-named files keep working with defaults — the init stays selected even while the file does not exist yet, and is picked up once written.