selfhost: retire stk[i].str spill workaround (closes #14)

End-to-end validation of #8: removed the task-#8 spill in
dotchainresolve; bootstrap ww2==ww3==ww4 still holds.

selfhost/cmd/w6c/main.combined.ww

@@ -7972,7 +7972,6 @@ export fn dotchainresolve(c: *cgen, n: *node, out: *dotchain) bool = {
     // bootstrap fixed-point (tests 993/995) — letting these locals get
     // declared inside the branch bodies trips a per-stage divergence in
     // slot counting.
-    let stepnd: *node = nil;
     let stepnm: str = "";
     let fi: *fieldinfo = nil;
     let found: *fieldinfo = nil;
@@ -7984,17 +7983,8 @@ export fn dotchainresolve(c: *cgen, n: *node, out: *dotchain) bool = {
     for (i >= 0) {
         let csi: *structinfo = structlookup(c, curstruct);
         if (csi == nil) { return false; };
-        // Spill `stk[i]` to a *node local before reading its `.str`.
+        if (stk[i] == nil) { return false; };
-        // Task #8 closed the cgen bug behind this — cstage's N_DOT
+        stepnm = stk[i].str;
-        // 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;
         fi = csi.fields;
         found = nil;
         for (fi != nil) {

selfhost/cmd/wcc/cgenutil.ww

@@ -2402,7 +2402,6 @@ export fn dotchainresolve(c: *cgen, n: *node, out: *dotchain) bool = {
     // bootstrap fixed-point (tests 993/995) — letting these locals get
     // declared inside the branch bodies trips a per-stage divergence in
     // slot counting.
-    let stepnd: *node = nil;
     let stepnm: str = "";
     let fi: *fieldinfo = nil;
     let found: *fieldinfo = nil;
@@ -2414,17 +2413,8 @@ export fn dotchainresolve(c: *cgen, n: *node, out: *dotchain) bool = {
     for (i >= 0) {
         let csi: *structinfo = structlookup(c, curstruct);
         if (csi == nil) { return false; };
-        // Spill `stk[i]` to a *node local before reading its `.str`.
+        if (stk[i] == nil) { return false; };
-        // Task #8 closed the cgen bug behind this — cstage's N_DOT
+        stepnm = stk[i].str;
-        // 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;
         fi = csi.fields;
         found = nil;
         for (fi != nil) {

selfhost/cmd/wwdump/main.combined.ww

@@ -7972,7 +7972,6 @@ export fn dotchainresolve(c: *cgen, n: *node, out: *dotchain) bool = {
     // bootstrap fixed-point (tests 993/995) — letting these locals get
     // declared inside the branch bodies trips a per-stage divergence in
     // slot counting.
-    let stepnd: *node = nil;
     let stepnm: str = "";
     let fi: *fieldinfo = nil;
     let found: *fieldinfo = nil;
@@ -7984,17 +7983,8 @@ export fn dotchainresolve(c: *cgen, n: *node, out: *dotchain) bool = {
     for (i >= 0) {
         let csi: *structinfo = structlookup(c, curstruct);
         if (csi == nil) { return false; };
-        // Spill `stk[i]` to a *node local before reading its `.str`.
+        if (stk[i] == nil) { return false; };
-        // Task #8 closed the cgen bug behind this — cstage's N_DOT
+        stepnm = stk[i].str;
-        // 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;
         fi = csi.fields;
         found = nil;
         for (fi != nil) {