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)

docs/BuildAndTest.md

@@ -32,7 +32,13 @@ make build
 make dev
 ```
 
-The output is a single binary: `buxc`.
+The output is a single binary: `buxc` (bootstrap compiler in Nim).
+
+The self-hosted compiler `buxc2` is built from `src_bux/*.bux` sources via:
+```bash
+make selfhost
+```
+This compiles `buxc2` using the bootstrap compiler. The self-hosted compiler generates C code and invokes `cc` to produce native binaries.
 
 ---
 
@@ -164,12 +170,18 @@ bux/
 ./buxc build -v
 ```
 
-### Inspecting Generated C
+### Inspecting Generated C (bootstrap)
 ```bash
 ./buxc build
 cat build/main.c
 ```
 
+### Inspecting Generated C (self-hosted)
+```bash
+cd _selfhost && ../buxc build
+cat build/main.c
+```
+
 ### Common Errors
 
 | Error | Cause | Fix |