feat(accessibility): roll HighContrastBorder out to tooltip + 3 panel borders

Continues the HC chrome rollout started by `c9af1ea` (which wired
just the modal scaffold). Tags four more static-border surfaces
so they boost to `BORDER_SUBTLE_HC` (#a0a0a0) when high-contrast
mode is on:

- **Tooltip** (`ui_tooltip.rs:191`). The hover-revealed caption
  popup. Border legibility matters because tooltips are usually
  brief — if the player has to squint to find the panel edge,
  the tooltip dismisses before they've parsed it.
- **Onboarding banner key chips** (`onboarding_plugin.rs:388`).
  The first-run UI's "press H or ?" key chips. First-run
  onboarding has the highest stakes for accessibility — a
  low-vision player who can't see the chips can't discover
  the help system.
- **Help panel key chips** (`help_plugin.rs:265`). Same
  treatment as the onboarding chips: keyboard-shortcut chips
  inside the F1 cheat sheet.
- **Stats panel cells** (`stats_plugin.rs:1019`). The S-key
  overlay's individual stat cells. A dense grid of bordered
  numbers is exactly the kind of surface where HC's
  `#505050 → #a0a0a0` boost makes the layout legible.

Each tagging is one line on the spawn tuple plus an import. The
existing `update_high_contrast_borders` system in
`settings_plugin` (added in `c9af1ea`) handles all tagged
entities uniformly — no system changes needed.

### Skipped on this pass

Sites with dynamic hover/focus paint systems (HUD action
buttons, modal buttons, radial menu rim) intentionally not
tagged because their existing paint cycles would race the HC
system. Wiring HC into those needs a different shape — either
fold HC into the dynamic-paint logic, or have HC consult the
hover/focus state. Future scope.

Other HC-tagging candidates (`home_plugin.rs:842/945/1158` home
menu element borders, `settings_plugin.rs:1952/2093/2214/2274`
settings panel rows) are likely fine to tag but I'm capping
this commit at four to keep it reviewable. Pattern is
established; future commits can extend.

1194 passing / 0 failing across the workspace (unchanged — no
new tests; the system-level test in `c9af1ea`'s scaffolding
covers all tagged entities uniformly). Workspace clippy clean.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

solitaire_engine/src/help_plugin.rs

@@ -14,8 +14,8 @@ use crate::ui_modal::{
     ScrimDismissible,
 };
 use crate::ui_theme::{
-    Z_MODAL_PANEL, BORDER_SUBTLE, RADIUS_SM, SPACE_2, TEXT_PRIMARY, TEXT_SECONDARY, TYPE_BODY,
-    TYPE_CAPTION, VAL_SPACE_1, VAL_SPACE_2, VAL_SPACE_3,
+    BORDER_SUBTLE, HighContrastBorder, RADIUS_SM, SPACE_2, TEXT_PRIMARY, TEXT_SECONDARY, TYPE_BODY,
+    TYPE_CAPTION, VAL_SPACE_1, VAL_SPACE_2, VAL_SPACE_3, Z_MODAL_PANEL,
 };
 
 /// Marker on the help overlay root node.
@@ -263,6 +263,7 @@ fn spawn_help_screen(commands: &mut Commands, font_res: Option<&FontResource>) {
                                 ..default()
                             },
                             BorderColor::all(BORDER_SUBTLE),
+                            HighContrastBorder::with_default(BORDER_SUBTLE),
                         ))
                         .with_children(|chip| {
                             chip.spawn((