selfhost/cmd: astrndup → strings.dup view (γ-2); drop wcc.astrndup

The final 2 astrndup callers in w6a (main.ww fname capture and the
dupstr wrapper in parse.ww) now use the uniform γ-1 shape:

    let view: str;
    view.ptr = src;
    view.len = n: i32;
    out = strings.dup(view);

With both call sites converted, wcc.astrndup is dead and removed
from selfhost/cmd/wcc/mem.ww. amalloc + arena bootstrap stay
(other callers; #7 Phase B/C territory).

The two `// astrndup until #11 (w6a types shadow) is fixed.`
WHY-pointers are obsolete (#11 landed in 6696e95) and dropped per
CLAUDE.md rule 8.

dupstr in parse.ww keeps its (*arena, *u8, u64) signature; the
vestigial *arena param is tracked by task #10.

Verified 132/132 incl. 991_w6a_ww + 995_self_rebuild byte-identity.

selfhost/cmd/w6l/main.combined.ww

@@ -833,24 +833,6 @@ export fn amalloc(a: *arena, n: u64) *void = {
 	return p: *void;
 };
 
-// astrndup — copy `n` bytes into the arena and produce a NUL-terminated
-// view. Returns a `str` whose ptr is arena-owned and whose len is `n`
-// (the trailing NUL is past `len`, so callers reading exactly n bytes
-// see no padding). Used by the lexer to capture token text.
-export fn astrndup(a: *arena, src: *u8, n: u64) str = {
-	let p: *u8 = amalloc(a, n + 1u64): *u8;
-	let i: u64 = 0u64;
-	for (i < n) {
-		p[i] = src[i];
-		i += 1u64;
-	};
-	p[n] = 0u8;
-	let r: str;
-	r.ptr = p;
-	r.len = n: i32;
-	return r;
-};
-
 export fn freearena(a: *arena) void = {
 	for (a != nil) {
 		let next: *arena = a.next;