Add headless JSON CLI

[plyrthn]

What this does

Adds non-interactive subcommands so shurectl can be scripted. Bare shurectl still launches the TUI; with a subcommand it prints JSON and exits.

Commands

• shurectl get - full device state as JSON
• shurectl set <setting> <value> - apply one setting, print the resulting state
• shurectl set help - list every setting, its accepted values, and the models it applies to
• shurectl preset list|save N|load N|delete N - host-side presets

Output is JSON on stdout. On error it prints {"error": ...} and exits non-zero, so callers can branch on the exit code.

shurectl set gain 24
shurectl set mute on
shurectl set hpf hz75
shurectl set compressor medium

Changes

• headless.rs (new) - command dispatch, value parsing, settings catalog
• main.rs - get/set/preset subcommands via clap; apply_preset_to_device refactored to take &ShureDevice so the TUI preset-load and preset load share it
• Cargo.toml - add serde_json

Design

• get reuses PresetSlot for its body, so the JSON field names and value tokens match the on-disk preset format
• set parses enum values into the existing Ser* mirror types, so the tokens get emits are exactly the tokens set accepts, and an unknown value reports the valid variants
• one catalog() is the source of truth for set help and the per-model applicability check, so a setting that does not apply to the connected model (e.g. phantom on an MV6) errors instead of silently no-opping
• no protocol changes; it calls the same device.rs methods the TUI does

Tested

Against an MV6: get/set/preset round-trip, the help catalog, and the applicability and value-validation errors. Adds 10 unit tests for the parsers and catalog/dispatch consistency; fmt and clippy clean.