lib/fmt: port V-side modifier formatting (#94 fold-e6)

V's vfprintf parsed mods via scanmods but dropped them after parse —
vformatfield routed straight to vwriteone (no width / alignment / pad
/ sign / base / prec honoured). Port the OLD modifier path (fmt.ww:
443-641 rawlen* / formatraw / formatone) into vstream.ww as the v*
twins, widen vformatfield to take *mods, and pass &m through vfprintf
at the call site.

The v* helpers mirror OLD verbatim (compute body identical; vputbytes
+ (size | io.error) routing replacing putbytes + (i32 | io.closed));
shared compute helpers (signof / digitsu64 / basenum) and modifier
enums (neg / alignment / mods) are reused directly from fmt.ww via
package scope. fmt.ww UNCHANGED — fold-eFinal (#50) collapses both
surfaces and dedupes the rawlen-family.

drew NaN/Inf signoff: strconv.f64tos / f32tos already render
"nan"/"infinity" with no leading '-', so the sign-peel in vrawlenf64
+ vformatraw f64 arm is a no-op on those views (same OLD path at
fmt.ww:520-562).

ken cs==ww mechanical: both stages compile the new V-side identically;
990-997 byte-id gates + combined_ww_fresh stay green (fmt is not
embedded in any selfhost main.combined.ww — grep verified pre-impl).

test/wcc/780_fmt_vstream_mods_run.c (cstage-only per #209): 5 rows
covering width / precision / base_hex / sign_plus / zero_pad. STAGE_WW
blocked by #209 (wwstage formattable match-arm bail), same carve-out
as 777_fmt_vstream_run.

Makefile

@@ -328,6 +328,7 @@ TESTS	= $(BIN)/test_smoke $(BIN)/test_lex $(BIN)/test_parse $(BIN)/test_check \
 	  $(BIN)/test_io_vtable_run \
 	  $(BIN)/test_memio_vstream_run \
 	  $(BIN)/test_fmt_vstream_run \
+	  $(BIN)/test_fmt_vstream_mods_run \
 	  $(BIN)/test_bufio_vstream_run \
 	  $(BIN)/test_log_vstream_run \
 	  $(BIN)/test_use_promote_alias \
@@ -706,6 +707,13 @@ $(BIN)/test_fmt_vstream_run: test/wcc/777_fmt_vstream_run.c \
 		$(LIB)/libwwrt.a | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<
 
+$(BIN)/test_fmt_vstream_mods_run: test/wcc/780_fmt_vstream_mods_run.c \
+		$(BIN)/ww $(BIN)/w6c $(BIN)/w6a $(BIN)/w6l \
+		$(BIN)/ww_ww $(BIN)/w6c_ww $(BIN)/w6a_ww $(BIN)/w6l_ww \
+		lib/fmt/fmt.ww lib/fmt/vstream.ww \
+		$(LIB)/libwwrt.a | $(BIN)
+	$(CC) $(CFLAGS) -o $@ $<
+
 $(BIN)/test_bufio_vstream_run: test/wcc/778_bufio_vstream_run.c \
 		$(BIN)/ww $(BIN)/w6c $(BIN)/w6a $(BIN)/w6l \
 		$(BIN)/ww_ww $(BIN)/w6c_ww $(BIN)/w6a_ww $(BIN)/w6l_ww \