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/ast.nim

@@ -127,6 +127,7 @@ type
     ekBorrow        ## borrow &mut expr — explicit borrow expression
     ekBlock
     ekMatch
+    ekStringInterp
 
   MatchArm* = object
     loc*: SourceLocation
@@ -219,6 +220,9 @@ type
     of ekMatch:
       exprMatchSubject*: Expr
       exprMatchArms*: seq[MatchArm]
+    of ekStringInterp:
+      exprInterpTexts*: seq[string]
+      exprInterpExprs*: seq[Expr]
 
   # ---------------------------------------------------------------------------
   # Statements
@@ -458,3 +462,6 @@ proc newBinaryExpr*(op: TokenKind, left, right: Expr, loc: SourceLocation): Expr
 
 proc newUnaryExpr*(op: TokenKind, operand: Expr, loc: SourceLocation): Expr =
   result = Expr(kind: ekUnary, loc: loc, exprUnaryOp: op, exprUnaryOperand: operand)
+
+proc newStringInterpExpr*(texts: seq[string], exprs: seq[Expr], loc: SourceLocation): Expr =
+  result = Expr(kind: ekStringInterp, loc: loc, exprInterpTexts: texts, exprInterpExprs: exprs)