feat: string interpolation via f"..." syntax

Bootstrap compiler:
- Add ekStringInterp AST node with text/expr parts
- Lexer: recognize f"..." prefix for interpolation strings
- Parser: split f"..." into ekStringInterp, parse inner expressions
  via sub-lexer/sub-parser. Supports escaped braces \{ and \}.
- Sema: type-check interpolated expressions, return String type
- HIR lowerer: desugar ekStringInterp to chained String_Concat
  calls with automatic type conversions:
  - int/uint types -> String_FromInt
  - float types -> String_FromFloat
  - bool -> String_FromBool
  - String -> as-is

Selfhost compiler:
- ast.bux: add ekStringInterp constant (reserved for future)
- lexer.bux: accept f" prefix, treat as plain string literal
  (selfhost parser does not implement interpolation yet)

Tests:
- _test_string_interp verifies int, float, bool, String,
  multiple interpolations, plain strings, and Fmt-style {0} templates

src/ast.bux

@@ -95,6 +95,7 @@ const ekBlock: int = 21;
 const ekMatch: int = 22;
 const ekSpawn: int = 24;
 const ekAwait: int = 25;
+const ekStringInterp: int = 26;
 
 struct ExprList {
     expr: *Expr,