test: migrate dotfield/idxfield compound-assign to @test + runww rejects, retire C twins (fold-3)

949_dotfield_compound + 949_idxfield_compound are one bug class (#133-lineage compound-assign load-op-store on field lvalues; #34/#33, #263 carve-out) sharing the combine + hard-error path, so the two C carriers fuse into one commit: 26 value rows -> test/lang @test row-tables (primitive-only asserts), 8 reject rows -> runww //ww:error dual-stage carriers. byteid floor 50->52.

test/wcc/data/dotfield_compound_float/case.ww

@@ -0,0 +1,14 @@
+//ww:error "single-dot field compound on float field"
+// Lifted from test/wcc/949_dotfield_compound_run.c reject row "dotfld_he_float"
+// (#34/rule-7): a compound assign on a single-dot FLOAT field is unwired in
+// both stages and MUST loud-fail, not silently demote to a plain store. The
+// substring is the shared diagnostic body — byte-identical on both stages,
+// pinned to "float field" so it cannot also match the str/slice/tagged rows
+// (no file:line, no cstage "ww: " prefix — both differ cs vs ww).
+package main;
+type S = struct { f: f64, g: int };
+export fn main() i32 = {
+	let s: S = S{f=1.0, g=0};
+	s.f *= 2.0;
+	return 0;
+};

test/wcc/data/dotfield_compound_slice/case.ww

@@ -0,0 +1,15 @@
+//ww:error "single-dot field compound on slice field"
+// Lifted from test/wcc/949_dotfield_compound_run.c reject row "dotfld_he_slice"
+// (#34/rule-7): a compound assign on a single-dot SLICE field is unwired in
+// both stages and MUST loud-fail. The substring is the shared diagnostic body
+// — byte-identical on both stages, pinned to "slice field" so it cannot also
+// match the float/str/tagged rows (no file:line, no cstage "ww: " prefix —
+// both differ cs vs ww).
+package main;
+type S = struct { f: []u8, g: int };
+export fn main() i32 = {
+	let b: [2]u8 = [1u8, 2u8];
+	let s: S = S{f=b[0:2], g=0};
+	s.f += b[0:1];
+	return 0;
+};

test/wcc/data/dotfield_compound_str/case.ww

@@ -0,0 +1,14 @@
+//ww:error "single-dot field compound on str field"
+// Lifted from test/wcc/949_dotfield_compound_run.c reject row "dotfld_he_str"
+// (#34/rule-7): a compound assign on a single-dot STR field is unwired in both
+// stages and MUST loud-fail. The substring is the shared diagnostic body —
+// byte-identical on both stages, pinned to "str field" so it cannot also match
+// the float/slice/tagged rows (no file:line, no cstage "ww: " prefix — both
+// differ cs vs ww).
+package main;
+type S = struct { f: str, g: int };
+export fn main() i32 = {
+	let s: S = S{f="a", g=0};
+	s.f += "x";
+	return 0;
+};

test/wcc/data/dotfield_compound_tagged/case.ww

@@ -0,0 +1,14 @@
+//ww:error "single-dot field compound on tagged field"
+// Lifted from test/wcc/949_dotfield_compound_run.c reject row "dotfld_he_tagged"
+// (#34/rule-7): a compound assign on a single-dot TAGGED-union field is unwired
+// in both stages and MUST loud-fail. The substring is the shared diagnostic
+// body — byte-identical on both stages, pinned to "tagged field" so it cannot
+// also match the float/str/slice rows (no file:line, no cstage "ww: " prefix —
+// both differ cs vs ww).
+package main;
+type S = struct { f: (i32|str), g: int };
+export fn main() i32 = {
+	let s: S = S{f=1, g=0};
+	s.f += 3;
+	return 0;
+};

test/wcc/data/idxfield_compound_float/case.ww

@@ -0,0 +1,14 @@
+//ww:error "arr[i].field compound on float field"
+// Lifted from test/wcc/949_idxfield_compound_run.c reject row "fld_he_float"
+// (#33/rule-7): a compound assign on an indexed-element FLOAT field is unwired
+// in both stages and MUST loud-fail, not silently fall through. The substring
+// is the shared diagnostic body — byte-identical on both stages, pinned to
+// "float field" so it cannot also match the str/slice/tagged rows (no
+// file:line, no cstage "ww: " prefix — both differ cs vs ww).
+package main;
+type S = struct { f: f64, g: i32 };
+export fn main() i32 = {
+	let xs: [2]S = [S{f=1.0, g=0}, S{f=0.0, g=0}];
+	xs[0].f /= 2.0;
+	return 0;
+};

test/wcc/data/idxfield_compound_slice/case.ww

@@ -0,0 +1,15 @@
+//ww:error "arr[i].field compound on slice field"
+// Lifted from test/wcc/949_idxfield_compound_run.c reject row "fld_he_slice"
+// (#33/rule-7): a compound assign on an indexed-element SLICE field is unwired
+// in both stages and MUST loud-fail. The substring is the shared diagnostic
+// body — byte-identical on both stages, pinned to "slice field" so it cannot
+// also match the float/str/tagged rows (no file:line, no cstage "ww: " prefix
+// — both differ cs vs ww).
+package main;
+type S = struct { f: []u8, g: i32 };
+export fn main() i32 = {
+	let b: [2]u8 = [1u8, 2u8];
+	let xs: [2]S = [S{f=b[0:2], g=0}, S{f=b[0:2], g=0}];
+	xs[0].f += b[0:1];
+	return 0;
+};

test/wcc/data/idxfield_compound_str/case.ww

@@ -0,0 +1,14 @@
+//ww:error "arr[i].field compound on str field"
+// Lifted from test/wcc/949_idxfield_compound_run.c reject row "fld_he_str"
+// (#33/rule-7): a compound assign on an indexed-element STR field is unwired in
+// both stages and MUST loud-fail. The substring is the shared diagnostic body
+// — byte-identical on both stages, pinned to "str field" so it cannot also
+// match the float/slice/tagged rows (no file:line, no cstage "ww: " prefix —
+// both differ cs vs ww).
+package main;
+type S = struct { f: str, g: i32 };
+export fn main() i32 = {
+	let xs: [2]S = [S{f="a", g=0}, S{f="b", g=0}];
+	xs[0].f += "x";
+	return 0;
+};

test/wcc/data/idxfield_compound_tagged/case.ww

@@ -0,0 +1,14 @@
+//ww:error "arr[i].field compound on tagged field"
+// Lifted from test/wcc/949_idxfield_compound_run.c reject row "fld_he_tagged"
+// (#33/rule-7): a compound assign on an indexed-element TAGGED-union field is
+// unwired in both stages and MUST loud-fail. The substring is the shared
+// diagnostic body — byte-identical on both stages, pinned to "tagged field" so
+// it cannot also match the float/str/slice rows (no file:line, no cstage
+// "ww: " prefix — both differ cs vs ww).
+package main;
+type S = struct { f: (i32|str), g: i32 };
+export fn main() i32 = {
+	let xs: [2]S = [S{f=1, g=0}, S{f=2, g=0}];
+	xs[0].f += 3;
+	return 0;
+};