lib: α-batch-2 rt.malloc → alloc([], N)! (path/shlex/fmt/ostest)

Phase 0 #8 second α-batch. 7 sites: lib/path/path.ww `join` ×4,
lib/shlex/shlex.ww `dupstr`, lib/fmt/fmt.ww `asprintf` tight-copy,
lib/os/ostest.ww `test_alloc_free_roundtrip`. Same dup-pilot pattern
(4c07ef0, 47918d3): `alloc([], N)!` + `buf.len = N;` +
`return strings.frombytes(buf);`.

Side effects:
- path/shlex/fmt: import switches `rt` → `strings` (callers now
  reference `strings.frombytes`, not `rt.malloc` direct).
- ostest.ww: `import rt;` retained — the alloc builtin lowers to
  `CALL malloc(SB)` which resolves via rt's @symbol("rt_malloc")
  decl. Other files reach rt transitively via `import strings`;
  ostest only imports os, so it needs the explicit rt import.
- shlex stale comment "avoid strings dep" stripped — strings is now
  in scope.

Verified make test 132/132 + 995_self_rebuild byte-identity.
Advances #43.

lib/os/ostest.ww

@@ -97,22 +97,18 @@ fn streq(a: str, b: str) bool = {
 
 // ---- alloc/free: mmap-backed runtime allocator ----------------------
 //
-// Direct round-trip. memio's dynamic-buffer tests already exercise
-// os.alloc / os.free transitively; the row here pins the FFI shape
-// at the lib/os layer (write+read-back proves the returned page is
-// dereferenceable, not just non-nil).
+// Direct round-trip. Write-then-read-back proves the returned page is
+// dereferenceable. A miscompiled binding (wrong arg order, wrong ABI,
+// etc.) would either fault or return zero here.
 
 @test fn test_alloc_free_roundtrip() void = {
-	let p: *u8 = rt.malloc(4096u64): *u8;
-	if (p == nil: *u8) { fail(); };
-	// Write a sentinel at the head and tail of the page, read it
-	// back. A miscompiled binding (wrong arg order, wrong ABI, etc.)
-	// would either fault or return zero here.
+	let p: []u8 = alloc([], 4096u64)!;
+	p.len = 4096;
 	p[0] = 90u8;		// 0x5a
 	p[4095] = 165u8;	// 0xa5
 	if (p[0] != 90u8) { fail(); };
 	if (p[4095] != 165u8) { fail(); };
-	os.free(p: *void, 4096u64);
+	os.free(p.ptr: *void, 4096u64);
 };
 
 export fn main() i32 = {