w6c+selfhost: cgen N_DOT N_INDEX-lhs branch (closes #8)

cstage cmd/w6c/cgen.c gained the missing N_DOT N_INDEX-lhs branch.
Covers both [N]*Struct and [N]Struct via fldloadop. wwstage already
handled [N]*Struct since 7c75dd2; refactored to mirror cstage exactly
and added [N]Struct. The spill workaround in dotchainresolve stays
(Pike rule); task #14 retires it as a follow-up.

Wwstage cgassign N_DOT(N_INDEX,...) silent store-drop discovered in
scope, filed as task #16.

selfhost/cmd/wcc/cgenutil.ww

@@ -2414,11 +2414,14 @@ export fn dotchainresolve(c: *cgen, n: *node, out: *dotchain) bool = {
     for (i >= 0) {
         let csi: *structinfo = structlookup(c, curstruct);
         if (csi == nil) { return false; };
-        // Materialise the *node first; wwstage's cgen mis-emits the
-        // chained shape `stk[i].str` directly (task #8 — loses BX
-        // between the index load and the field deref), so always
-        // spill to an intermediate local before reading the str
-        // field. The cstage emits the same pattern for byte-identity.
+        // Spill `stk[i]` to a *node local before reading its `.str`.
+        // Task #8 closed the cgen bug behind this — cstage's N_DOT
+        // had no N_INDEX-lhs branch and fell through, returning the
+        // (AX, BX) cgindex shape as if it were the field. The fix
+        // lives in cmd/w6c/cgen.c N_DOT and the mirror in this
+        // module's cgenexpr.ww cgdot. The spill stays for now (Pike
+        // rule — workaround retirement is task #14), so cstage
+        // compiling this source emits byte-identical asm to wwstage.
         stepnd = stk[i];
         if (stepnd == nil) { return false; };
         stepnm = stepnd.str;