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

stdlib/Std/Fs.bux

@@ -0,0 +1,19 @@
+module Std::Fs {
+
+extern func bux_dir_exists(path: String) -> int;
+extern func bux_mkdir_if_needed(path: String) -> int;
+extern func bux_list_dir(dir: String, ext: String, out_count: *int) -> *String;
+
+func DirExists(path: String) -> bool {
+    return bux_dir_exists(path) != 0;
+}
+
+func Mkdir(path: String) -> bool {
+    return bux_mkdir_if_needed(path) != 0;
+}
+
+func ListDir(dir: String, ext: String, count: *int) -> *String {
+    return bux_list_dir(dir, ext, count);
+}
+
+}