Add formula engine: data-driven expression evaluator (Core)

Keystone for the gene system. A Formula compiles a string from a def
(lexer -> recursive-descent parser -> tree of Func<IFormulaContext,float>)
once, then evaluates allocation-free against a variable context.

Supports + - * / %, comparisons, && || !, ternary, the constants
pi/tau/e, and functions abs sign floor ceil round sqrt exp log sin cos
tan min max pow clamp lerp step. Deterministic and side-effect-free so
gene-effect formulas stay pure. Covered by FormulaTests (parsing,
precedence, functions, logic/ternary, variables, errors).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/MrGameEng.Core/Formulas/FormulaException.cs

@@ -0,0 +1,12 @@
+namespace MrGameEng.Formulas;
+
+/// <summary>
+/// Thrown when a <see cref="Formula"/> cannot be lexed or parsed, or when a compiled formula
+/// references a variable the context cannot resolve at evaluation time.
+/// </summary>
+public sealed class FormulaException : Exception
+{
+    /// <summary>Creates the exception with a human-readable <paramref name="message"/>.</summary>
+    public FormulaException(string message)
+        : base(message) { }
+}