fix: multi-arg calls, uninitialized symbols, param limit; add banner img; update docs

- Fix segfault from uninitialized Symbol structs in scope/hir_lower/sema
- Fix null ReadFile crash in Cli_MergeFileInto (missing stdlib files)
- Extend function params from 6 to 9 (bux_str_format needs 9 args)
- Add ExprList/HirArgList linked lists for >2 call arguments
- Add banner image to README
- Update docs: C backend is now primary (not QBE)

src_bux/hir.bux

@@ -34,6 +34,15 @@ const hMatch: int = 28;
 const hSpawn: int = 29;
 const hAwait: int = 30;
 
+// ---------------------------------------------------------------------------
+// HirArgList — linked list for call arguments beyond 2
+// ---------------------------------------------------------------------------
+
+struct HirArgList {
+    node: *HirNode,
+    next: *HirArgList,
+}
+
 // ---------------------------------------------------------------------------
 // HirNode — unified struct with tagged union pattern
 // ---------------------------------------------------------------------------
@@ -76,6 +85,9 @@ struct HirFunc {
     param3: HirParam;
     param4: HirParam;
     param5: HirParam;
+    param6: HirParam;
+    param7: HirParam;
+    param8: HirParam;
     retTypeKind: int;
     retTypeName: String;
     body: *HirNode;