docs: update README, Stdlib, BuildAndTest, PLAN for new std modules

Added documentation for:
- Std::Fs (DirExists, Mkdir, ListDir)
- Std::Mem (Alloc, Free, MemEq, New)
- Std::Set<T> (Set_New, Set_Add, Set_Has)
- String_Find, String_Replace, String_Format

Updated PLAN.md blockers (all resolved) and self-host status.
Updated README.md project structure and feature table.
Updated BuildAndTest.md with new stdlib modules and path syntax.

Also includes:
- CLI path argument fix (build/check/run accept project path)
- Remove dummyFunc from parser
- Fix String_Replace to use String_Concat instead of buggy str_join2

src_bux/parser.bux

@@ -298,10 +298,6 @@ func parserParsePrimary(p: *Parser) -> *Expr {
 }
 
 // ---------------------------------------------------------------------------
-
-func dummyFunc() -> int {
-    return 0;
-}
 // Postfix: call, index, field access, as, is, ?
 // ---------------------------------------------------------------------------