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

bootstrap/lexer.nim

@@ -527,7 +527,10 @@ proc nextToken(lex: var Lexer): Token =
     discard lex.advance()
     return Token(kind: tkNewLine, text: "\n", loc: startLoc)
 
-  # String prefixes: c8" c16" c32" — must come before ident check
+  # String prefixes: f" c8" c16" c32" — must come before ident check
+  if c == 'f' and lex.peek(1) == '"':
+    discard lex.advance()  # f
+    return lex.scanString(startLoc, 1)
   if c == 'c' and lex.peek(1) in {'8', '1', '3'}:
     let d = lex.peek(1)
     if d == '8' and lex.peek(2) == '"':