feat(player): #6 fold enchantment buffs into vital max via EnchantmentMath

Ports CEnchantmentRegistry::EnchantAttribute (PDB 0x00594570, see
docs/research/named-retail/acclient_2013_pseudo_c.txt line 416110).
The retail formula:

    real_max = (vital.(ranks+start) + attribute_contribution) * mult_buff + add_buff
    clamp >= 5 if base >= 5 else >= 1

is now applied in LocalPlayerState.GetMaxApprox.

EnchantmentMath.GetMod(activeEnchantments, table, statKey)
  - Family-stacking dedup via SpellTable.Family (only one buff per
    family-bucket wins, by highest spell-id as a generation proxy).
  - Family=0 means "no bucket" — each layer is its own bucket.
  - Returns (Multiplier, Additive) ready to apply.
  - StatKey constants: MaxHealth=1, MaxStamina=3, MaxMana=5
    (verified against named-retail/acclient.h line 37287-37301).

Spellbook.GetVitalMod(statKey) delegates to EnchantmentMath using
its constructor-injected SpellTable.

LocalPlayerState.GetMaxApprox now applies the full formula with
the min-vital floor (matches CreatureVital::GetMaxValue at PDB
0x0058F2DD). When Spellbook is null (back-compat), falls back to
Identity (no buff modification) — existing tests stay green.

GameWindow constructor wires SpellBook -> LocalPlayer so the chain
is complete in the live session.

Architecture in place; data still flat.

Until ISSUES.md #12 lands the wire-format extension that captures
StatMod (type/key/val) on ActiveEnchantmentRecord, the per-enchantment
modifier value isn't aggregated yet — GetMod returns Identity. Once
#12 wires the data, the existing aggregator + formula light up
automatically. Live +Acdream Stam/Mana will keep reading ~95% until
#12 lands.

6 new EnchantmentMathTests cover: empty list returns Identity,
no-table-entries returns Identity, stat-key constants match ACE,
Identity is (1, 0), family-stacking dedup, family=0 (no-bucket).

Total tests: 828 -> 834.

Closes #6 architecturally. Files #12 to track the wire-data follow-up.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/AcDream.Core/Spells/Spellbook.cs

@@ -46,6 +46,17 @@ public sealed class Spellbook
     /// Returns <see cref="SpellTable.Empty"/> if none was provided.</summary>
     public SpellTable Metadata => _table;
 
+    /// <summary>
+    /// Issue #6 — combined buff modifier for a vital stat. Aggregates
+    /// over <see cref="ActiveEnchantments"/> through
+    /// <see cref="EnchantmentMath.GetMod"/> with family-stacking dedup
+    /// from the spell metadata table. Returns
+    /// <see cref="EnchantmentMath.VitalMod.Identity"/> when no buffs
+    /// apply (or when no SpellTable was wired).
+    /// </summary>
+    public EnchantmentMath.VitalMod GetVitalMod(uint statKey) =>
+        EnchantmentMath.GetMod(ActiveEnchantments, _table, statKey);
+
     /// <summary>Fires when a spell is added to the player's spellbook.</summary>
     public event Action<uint>? SpellLearned;