feat: backtick raw/multi-line strings + QBE backend fixes

- Add backtick raw string literals (Go-style ): no escape processing, multi-line
- C backend: handle backtick strings in emitExpr
- Self-hosted lexer: lexScanBacktickString support
- QBE backend: handle backtick strings in data sections
- QBE: remove broken extern declarations (QBE resolves at link time)
- QBE: fix comparison result types (ceql → w, extend to l)
- QBE: fix SSA parameter name clash (_p_ prefix for params)
- QBE: const inlining via HirLower + QBE emitter
- QBE: proper type mapping for bool/char types
- hir_lower: collect dkConst declarations for const propagation

src/token.nim

@@ -5,7 +5,7 @@ type
     ##Literals
     tkIntLiteral      # 42  0xFF  0b1010  0o77
     tkFloatLiteral    # 3.14  1.0e-9
-    tkStringLiteral   # "hello"  c8"hello"  c16"hello"  c32"hello"
+    tkStringLiteral   # "hello"  c8"hello"  c16"hello"  c32"hello"  `raw multi-line`
     tkCharLiteral     # 'A'  c8'A'  c16'A'  c32'A'
     tkBoolLiteral     # true  false