M6.18: Skill expansion to 5 per ability — 12 new Theriapolis-flavored skills

Skill list grows from 18 → 30 so every ability has 5 skills tied to it.
New skills appended at SkillId byte indices 18–29 to preserve
save-game compat with pre-M6.18 characters.

  STR (+4): Brawl, Build-Read, Force, Haulage
  DEX (+2): Driving, Marksmanship
  CON (+5): Endurance, Fortitude, Hardiness, Lung-Craft, Pain Tolerance
  CHA (+1): Scent-Speak

Endurance covers applied effort over time; Hardiness covers external
condition tolerance (temperature, smoke, altitude); Fortitude is
ingestion-only (poison, spoiled food, ritual draughts). Pain Tolerance
is function-while-wounded. Lung-Craft is breath/voice discipline.

Class skill_options updated across all 8 callings so every new skill
has 3+ class homes (Muzzle-Speaker remains the universal pick of
all 30). Backgrounds left untouched — their skill grants are
doc-canon and would benefit from a separate balancing pass.

CharacterBuilder.SkillToJsonName extended with explicit cases for the
new skills so compound enum names like BuildRead serialize as
"build_read" rather than the fallback "buildread" — caught by
CharacterBuilderTests.

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

Theriapolis.Core/Rules/Character/CharacterBuilder.cs

@@ -401,6 +401,19 @@ public sealed class CharacterBuilder
         SkillId.SleightOfHand   => "sleight_of_hand",
         SkillId.Stealth         => "stealth",
         SkillId.Survival        => "survival",
+
+        SkillId.Brawl           => "brawl",
+        SkillId.BuildRead       => "build_read",
+        SkillId.Driving         => "driving",
+        SkillId.Endurance       => "endurance",
+        SkillId.Force           => "force",
+        SkillId.Fortitude       => "fortitude",
+        SkillId.Hardiness       => "hardiness",
+        SkillId.Haulage         => "haulage",
+        SkillId.LungCraft       => "lung_craft",
+        SkillId.Marksmanship    => "marksmanship",
+        SkillId.PainTolerance   => "pain_tolerance",
+        SkillId.ScentSpeak      => "scent_speak",
         _ => s.ToString().ToLowerInvariant(),
     };