w6c+selfhost: cgen *T-rooted chained N_DOT (closes #22)

Spine walker now accepts *T root at the last hop (cur->lhs->kind ==
N_IDENT, pu->kind == TY_PTR), substitutes pointee struct, emits
MOVQ off(BP), CX before offset arithmetic. Symmetric in N_DOT and
N_ASSIGN. Distinct gate from existing mid-chain *T-field branch
(cgen.c:4615) — no shadow.

Unblocks task #18 (bufio writer first-field embed). Pre-existing
*T-field mid-chain path unchanged.

selfhost/cmd/wcc/cgenutil.ww

@@ -2364,10 +2364,11 @@ fn cgwidentaggedstore(c: *cgen, dst: *node, src: *node, slot_off: i32, slot_sz:
 export fn dotchainresolve(c: *cgen, n: *node,
     outrootname: *str, outrootoff: *i32, outtotaloff: *i32,
     outleaffi: **fieldinfo, outslicedelta: *i32,
-    outisglobal: *bool) bool = {
+    outisglobal: *bool, outptrroot: *bool) bool = {
     *outrootname = "";
     *outrootoff = 0;
     *outisglobal = false;
+    *outptrroot = false;
     *outtotaloff = 0;
     *outleaffi = nil;
     *outslicedelta = -1;
@@ -2395,6 +2396,20 @@ export fn dotchainresolve(c: *cgen, n: *node,
                 rootstruct = lc.tnode.str;
                 *outrootoff = lc.off;
             };
+            // `*T` root (param/local): dereference at emit time;
+            // pointee struct supplies the field layout. Callers
+            // that opt in via *outptrroot emit a MOVQ load of the
+            // slot before indexing.
+            if (lc.tnode.kind == nkind.N_TPTR) {
+                let pe: *node = lc.tnode.lhs;
+                if (pe != nil) {
+                    if (pe.kind == nkind.N_TNAME) {
+                        rootstruct = pe.str;
+                        *outrootoff = lc.off;
+                        *outptrroot = true;
+                    };
+                };
+            };
         };
     };
     if (rootstruct.len == 0) {