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/cli.nim

@@ -258,7 +258,7 @@ proc collectDepDecls(lock: Lockfile, root: string, opts: GlobalOptions): seq[Dec
 
 proc cmdCheck*(args: seq[string], opts: GlobalOptions): int =
   let useColor = shouldUseColor(opts)
-  let root = getCurrentDir()
+  let root = if args.len > 0: absolutePath(args[0]) else: getCurrentDir()
   let manifestPath = root / "bux.toml"
   if not fileExists(manifestPath):
     printError("no bux.toml found", useColor)
@@ -415,7 +415,7 @@ proc mergeDecls(stdlibDecls: seq[Decl], userDecls: seq[Decl]): seq[Decl] =
 
 proc cmdBuild*(args: seq[string], opts: GlobalOptions): int =
   let useColor = shouldUseColor(opts)
-  let root = getCurrentDir()
+  let root = if args.len > 0: absolutePath(args[0]) else: getCurrentDir()
   let manifestPath = root / "bux.toml"
   if not fileExists(manifestPath):
     printError("no bux.toml found", useColor)
@@ -554,7 +554,7 @@ proc cmdRun*(args: seq[string], opts: GlobalOptions): int =
   let buildRes = cmdBuild(args, opts)
   if buildRes != 0:
     return buildRes
-  let root = getCurrentDir()
+  let root = if args.len > 0: absolutePath(args[0]) else: getCurrentDir()
   let man = loadManifest(root / "bux.toml")
   let outputName = if man.name != "": man.name else: "bux_out"
   let outputFile = root / "build" / outputName