v2: Implement Interpreter.atg with typed FunctionValue, mixed-arg functions, and expression-binding (:=); fix Coco/R csproj command

This completes the v2 grammar based on master’s capabilities while aligning with the v2 design philosophy (numeric-first, strings as function-args only):

- Implement full arithmetic grammar ( +, -, *, /, %, ^, parentheses, unary sign ), with support for number/hex/binary literals.
- Add variables and arrays:
  - Numeric variables via :: (evaluate RHS now and store numeric)
  - Expression-bound variables via := (capture RHS text; re-parse and evaluate at use-time)
  - Numeric arrays via -> and array literals + indexing (with bounds/type checks)
- Add comparisons (==, <, >, <=, >=) producing FunctionValue TRUE/FALSE.
- Add function calls with mixed arguments: numeric expressions and quoted string literals.
  - In numeric contexts, enforce numeric results; emit clear type errors if a function returns a string.
- Root production always produces a FunctionValue through InputInterpreter.ProduceOutput(FunctionValue).

Runtime integration (expected APIs on InputInterpreter):
- Variable APIs: AssignNumeric, AssignExpression, AssignNumericArray; TryGetNumeric, TryGetExpression, TryGetNumericArray
- Function dispatch: ExecuteFunction(name, FunctionArgument[])
- Expression evaluation for :=: EvaluateExpression(expressionText)

Coco/R build fix:
- Correct the PreBuild command in src/core/core.csproj:
  - Use -frames (without the stray space) and point to cocor
  - Use the correct case and path for the grammar: cocor/Interpreter.atg

Notes:
- Strings are valid only as function arguments and not as standalone values or variables.
- Grammar emits concise, actionable error messages for type mismatches, missing variables, and array bounds.

src/core/core.csproj

@@ -8,7 +8,7 @@
   </PropertyGroup>
 
   <Target Name="PreBuild" BeforeTargets="PreBuildEvent">
-    <Exec Command="dotnet tool run coco -namespace csmic.Interpreter - frames . $(ProjectDir)\cocor\interpreter.atg" />
+    <Exec Command="dotnet tool run coco -namespace csmic.Interpreter -frames $(ProjectDir)cocor $(ProjectDir)cocor/Interpreter.atg" />
   </Target>
 
 </Project>