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>

solitaire_engine/src/hud_plugin/tests.rs

@@ -1,4 +1,5 @@
 use super::*;
+use crate::auto_complete_plugin::AutoCompleteState;
 use crate::game_plugin::GamePlugin;
 use crate::table_plugin::TablePlugin;
 use chrono::Local;