cstage+selfhost+test: fix (*p).f silent drop in N_DOT lhs (read+write)

Pre-existing landmine surfaced by #5 (whole-STRUCT N_ASSIGN). Both
stages' N_DOT dispatch gated on `lhs->lhs->kind == N_IDENT`; the
parser produces N_UN(STAR, IDENT(p)) for `(*p).f`, so both sides fell
off:
  - Write side (cgassign N_DOT base): emitted nothing, store dropped.
  - Read side (case N_DOT pointer-auto-deref): cgexpr derefed the
    pointer as a scalar, AX = first qword of struct, field offset
    dropped.

Fix: retarget base / dot_lhs to the inner IDENT when shape is
N_UN(STAR, IDENT). The existing via_ptr branch fires identically to
`p.f`. v1 scope is bare-IDENT inner only; `(*expr).f` (non-IDENT
pointer expression) is tracked separately as task #19.

702 covers 7 rows: write_i64/i32/str, read_i64/i32/str_len, roundtrip

Makefile

@@ -228,6 +228,7 @@ TESTS	= $(BIN)/test_smoke $(BIN)/test_lex $(BIN)/test_parse $(BIN)/test_check \
 	  $(BIN)/test_samemod_prefer \
 	  $(BIN)/test_cgreturn_struct \
 	  $(BIN)/test_cgassign_struct \
+	  $(BIN)/test_dot_explicit_deref \
 	  $(BIN)/test_use_promote_alias \
 	  $(BIN)/test_field_signed $(BIN)/test_frame_argcount \
 	  $(BIN)/test_selfhost $(BIN)/test_w6a_ww $(BIN)/test_w6l_ww \
@@ -381,6 +382,12 @@ $(BIN)/test_cgassign_struct: test/wcc/701_cgassign_struct.c \
 		$(LIB)/libwwrt.a | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<
 
+$(BIN)/test_dot_explicit_deref: test/wcc/702_dot_explicit_deref.c \
+		$(BIN)/ww $(BIN)/w6c $(BIN)/w6a $(BIN)/w6l \
+		$(BIN)/ww_ww $(BIN)/w6c_ww $(BIN)/w6a_ww $(BIN)/w6l_ww \
+		$(LIB)/libwwrt.a | $(BIN)
+	$(CC) $(CFLAGS) -o $@ $<
+
 $(BIN)/test_use_promote_alias: test/wcc/699_use_promote_alias.c \
 		$(BIN)/ww $(BIN)/w6c $(BIN)/w6a $(BIN)/w6l \
 		$(LIB)/libwwrt.a | $(BIN)