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

docs/BuildAndTest.md

@@ -71,12 +71,15 @@ Output = "Bin"
 ```bash
 # Type-check without building
 ./buxc check
+./buxc check ./myproject
 
 # Build
 ./buxc build
+./buxc build ./myproject
 
 # Build and run
 ./buxc run
+./buxc run ./myproject
 
 # Clean build artifacts
 ./buxc clean
@@ -136,7 +139,14 @@ bux/
 │   │   ├── Io.bux
 │   │   ├── Array.bux
 │   │   ├── String.bux
-│   │   └── Map.bux
+│   │   ├── Map.bux
+│   │   ├── Fs.bux
+│   │   ├── Mem.bux
+│   │   ├── Set.bux
+│   │   ├── Path.bux
+│   │   ├── Math.bux
+│   │   ├── Task.bux
+│   │   └── Channel.bux
 │   ├── runtime.c     # C runtime shim
 │   └── io.c          # C I/O functions
 ├── examples/         # Example programs