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_bux/lexer.bux

@@ -384,6 +384,20 @@ func lexScanNumber(lex: *Lexer) {
 // Strings and chars
 // ---------------------------------------------------------------------------
 
+func lexScanBacktickString(lex: *Lexer) {
+    lexMarkStart(lex);
+    if lexPeek(lex, 0) == 96 { discard lexAdvance(lex); } // opening backtick
+    while !lexIsAtEnd(lex) && lexPeek(lex, 0) != 96 {
+        discard lexAdvance(lex);
+    }
+    if lexIsAtEnd(lex) {
+        lexEmitDiag(lex, "unterminated backtick string literal");
+    } else {
+        discard lexAdvance(lex); // closing backtick
+    }
+    lexEmitToken(lex, tkStringLiteral);
+}
+
 func lexScanString(lex: *Lexer) {
     lexMarkStart(lex);
     if lexPeek(lex, 0) == 34 { discard lexAdvance(lex); } // opening "
@@ -618,6 +632,11 @@ func lexNextToken(lex: *Lexer) {
         lexScanString(lex); return;
     }
 
+    // Backtick raw string
+    if c == 96 { // backtick
+        lexScanBacktickString(lex); return;
+    }
+
     // Char prefixes: c8' c16' c32'
     if c == 99 { // 'c'
         let d: uint32 = lexPeek(lex, 1);