ci: optional selfhost-loop workflow + determinism script

Add tools/selfhost_loop.sh with path-normalized C and stripped ELF
checks (bootstrap ×2). Wire GitHub Actions (manual/weekly/main paths)
outside default make test. Raise selfhost lexer maxTokens for large files.

docs/BuildAndTest.md

@@ -189,6 +189,17 @@ summary table and exits:
 
 Use `Std::Test` module for assertions inside test code.
 
+### Selfhost loop (optional CI)
+```bash
+make selfhost-loop                 # bootstrap builds src/ twice; C+ELF match
+BUX_SELFHOST_FIXED_POINT=1 make selfhost-loop   # also buxc2→buxc3 (experimental)
+```
+Not part of default `make test`. GitHub Actions workflow
+`.github/workflows/selfhost-loop.yml` runs it on:
+- manual `workflow_dispatch`
+- weekly schedule
+- pushes to `main` that touch `src/`, `lib/`, bootstrap, or the loop script
+
 ### Format (`bux fmt`)
 ```bash
 ./buxc fmt examples/hello.bux  # reformat one file