M6.5: StepSkills + ability-bonus breakdown + Aside redesign

The skills step lands as the last data-driven character creation step
(only Sign — name + confirm — remains for M6.6). Brought a few cross-
cutting refactors with it.

Scenes/Steps/StepSkills.cs:
  Direct port of StepSkills in src/steps.jsx — all 18 skills laid out
  in 6 ability-grouped panels (STR/DEX/CON/INT/WIS/CHA), 2-column
  grid. Background-granted skills appear pre-checked and locked;
  user picks `class.SkillsChoose` more from `class.SkillOptions`.
  Hover the skill name → popover with the codex flavor description
  (limited to the title only — hovering checkboxes / source tags
  doesn't trigger the popover, avoids interference with adjacent
  rows' click targets). Fixed-width [✓] / [ ] / [—] checkbox slot
  so toggling doesn't shift the row layout.

UI/SkillsCatalog.cs (new):
  Static skill table — JSON id, display label, governing ability,
  and the codex SKILL_DESC text ported verbatim from src/data.jsx.
  Mirrors Theriapolis.Core.Rules.Stats.SkillId; descriptions live
  here because backgrounds.json and classes.json don't carry them.

UI/AbilityCalc.cs (new):
  Final-score math — base assignment + clade and species mods, with
  per-source breakdown for the bonus popover ("+1 from Canidae · +2
  from Wolf"). Hybrid mode tags each clade source with its lineage
  ("(sire)" / "(dam)"). Used by both StepStats and the Aside so the
  two views can never disagree on what a +N badge means.

UI/BackgroundAvailability.cs (new):
  Extracted from StepBackground — shared rules table for hybrid-only
  and clade-restricted backgrounds. Now also consulted by StepClade
  when the player changes lineage: the currently-selected background
  is auto-cleared if the new lineage no longer satisfies its rule
  (e.g., Pack-Raised clears when switching from Canidae to Felidae,
  Passer clears when toggling Hybrid off). Implemented via
  Resource.Duplicate + Patch on the duplicate to evaluate the
  hypothetical post-patch state without committing prematurely.

StepStats.cs:
  Per-row layout extended: ability label | slot | bonus chip | final
  | d20 mod. Bonus chip is a TraitChip with the per-source breakdown
  in its hover description. Auto-assign now sorts empty abilities by
  AbilityCalc.TotalBonus DESCENDING (with class.PrimaryAbility as
  tiebreaker) — biggest pool value lands on the ability already
  receiving the biggest lineage bonus, maximising final scores.

Aside.cs (significant redesign):
  - Name centered at top.
  - Lineage details: 2-column grid, full-width.
    - Purebred:  Clade | Species, then Calling | Background, then
                 Subclass | (empty).
    - Hybrid:    SIRE ★ | DAM (centered + underlined column heads),
                 Clade | Clade, Species | Species, then the same
                 calling/background/subclass rows.
  - Attributes: STR/DEX/CON/INT/WIS/CHA each with bonus chip (omitted
    when +0), final score, d20 modifier. Self-contained min-width
    table so it can't widen the panel beyond its alloc.
  - Pills: traits, detriments, level-1 features, background feature,
    skill chips (BG-locked + user-chosen). All hoverable for descriptions.
  - Whole panel wraps in a ScrollContainer so an over-tall summary
    scrolls in place instead of pushing the wizard layout off-screen.
  - Width nudged 320 → 360px. Smaller font on label tags, autowrap
    on value labels so long names ("Hybrid Underground") wrap rather
    than push the panel wider.

Card grids: changed all five card-grid steps (Clade, Species, Class,
Subclass, Background) from SizeFlagsHorizontal.ExpandFill →
ShrinkCenter. Cards stay at their CustomMinimumSize 200 wide and
the grid horizontally centers in PageMain. The right-side gap
between content and Aside is now uniform regardless of how many
cards or whether the last row is partial — fixes the "Clade tab
feels too padded, Background tab too tight" perception.

Closes M6.5. Per guide §12, what's left in M6: M6.6 (StepReview —
name + summary + Confirm handoff per guide §11) and M6.7 (parchment
Theme pass).

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

Theriapolis.Godot/Scenes/Steps/StepBackground.cs

@@ -43,7 +43,7 @@ public partial class StepBackground : VBoxContainer, IStep
             AutowrapMode = TextServer.AutowrapMode.WordSmart,
         });
 
-        _grid = new GridContainer { Columns = 3, SizeFlagsHorizontal = SizeFlags.ExpandFill };
+        _grid = new GridContainer { Columns = 3, SizeFlagsHorizontal = SizeFlags.ShrinkCenter };
         _grid.AddThemeConstantOverride("h_separation", 16);
         _grid.AddThemeConstantOverride("v_separation", 16);
         AddChild(_grid);
@@ -51,37 +51,13 @@ public partial class StepBackground : VBoxContainer, IStep
         Refresh();
     }
 
-    /// <summary>
-    /// Background-availability rules. The backgrounds.json schema doesn't
-    /// carry restriction fields — the gating lives in flavor text only —
-    /// so they're hardcoded here. Each predicate returns true when the
-    /// background is available to the given draft; missing entries are
-    /// universally available.
-    ///
-    /// If backgrounds.json ever gains structured restriction fields,
-    /// swap these out for a property-driven check.
-    /// </summary>
-    private static readonly System.Collections.Generic.Dictionary<string, System.Func<UI.CharacterDraft, bool>> AvailabilityRules = new()
-    {
-        // Hybrid-only backgrounds — flavor text explicitly hybrid.
-        { "passer",             d => d.IsHybrid },
-        { "hybrid_underground", d => d.IsHybrid },
-        { "former_chattel",     d => d.IsHybrid },
-
-        // Clade-restricted backgrounds.
-        { "warren_runner",      d => d.HasClade("leporidae") },
-        { "pack_raised",        d => d.HasClade("canidae") },
-        { "herd_city_born",     d => d.HasAnyCladeOfKind("prey") },
-    };
-
     private void Refresh()
     {
         if (_grid is null) return;
         foreach (var c in _grid.GetChildren()) c.QueueFree();
         foreach (var bg in CodexContent.Backgrounds)
         {
-            if (AvailabilityRules.TryGetValue(bg.Id, out var rule) && !rule(_draft))
-                continue;
+            if (!BackgroundAvailability.IsAvailable(bg.Id, _draft)) continue;
             _grid.AddChild(BuildCard(bg));
         }
     }