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>
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