refactor(engine): split hud_plugin runtime code into submodules
Continues #118 after settings_plugin (PR #127): the 2,725-line mod.rs
becomes five focused submodules along existing system boundaries —
mod.rs 553 markers, resources, popover enums, plugin build
spawn.rs 552 band / columns / avatar / action-bar construction
interaction.rs 616 button handlers, Modes/Menu popovers, tap gesture
fx.rs 430 action fades, score pulses/floaters, streak flourish
updates.rs 612 HUD text / typography / visibility updaters
Moved items are pub(super) (fx re-exported pub for HudActionFade and
format_time_limit consumers). Code moved verbatim; no behaviour change;
all 44 hud tests pass unchanged.
Refs #118
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
funman300
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