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/hir_lower.bux

@@ -412,6 +412,8 @@ func HirLower_LowerModule(mod: *Module, sema: *Sema) -> *HirModule {
     hm.funcs = ctx.funcs;
     hm.structCount = 0;
     hm.structs = bux_alloc(64 as uint * sizeof(HirStruct)) as *HirStruct;
+    hm.constCount = 0;
+    hm.consts = bux_alloc(512 as uint * sizeof(HirConst)) as *HirConst;
 
     // First pass: count structs (to allocate field arrays later)
     // Second pass: actually collect them
@@ -468,6 +470,18 @@ func HirLower_LowerModule(mod: *Module, sema: *Sema) -> *HirModule {
             }
             hm.structCount = hm.structCount + 1;
         }
+        if decl.kind == dkConst && hm.constCount < 512 {
+            let ci: int = hm.constCount;
+            hm.consts[ci].name = decl.strValue;
+            var val: int = 0;
+            if decl.constValue != null as *Expr {
+                if decl.constValue.kind == ekLiteral {
+                    val = decl.constValue.intValue;
+                }
+            }
+            hm.consts[ci].value = val;
+            hm.constCount = hm.constCount + 1;
+        }
         if decl.kind == dkEnum {
             hm.enumCount = hm.enumCount + 1;
         }