lib/os: graduate timespec to time.instant

Removes the local os.timespec (sec, nsec) struct in favour of
time.instant from lib/time. lib/os now `use time;`. filestat's
atime/mtime/ctime change type with byte-identical layout
(i64+i64=16B both sides), so .sec / .nsec accessors at all caller
sites work unchanged.

Rule 12: simple data + mirror Hare. Two same-layout types — one
Hare-canonical, one not — is exactly the structural divergence the
rule forbids. Single-source-of-truth; no transitional alias.

Citations: ref/hare/fs/types.ha:141 (Hare's fs::filestat carries
time::instant), ref/hare/time/instant.ha:9 (canonical layout).

Caller impact (sole reader): lib/os/stattest.ww (.sec / .nsec
unchanged; one comment line refreshed). examples/cmatrix migrated
already in 7e9bede. No selfhost/cmd/* reads mtime/atime/ctime.

Test wiring: lib/os/os.ww removed from 900_stdlib.c's standalone-
w6c-codegen list (cross-module type ref now needs the driver's
module concatenation, same reason lib/bufio and lib/fmt graduated
off earlier). Coverage stays at 976_stat_run via stattest.ww.
Makefile dep edges for the five wwstage targets gain
lib/time/time.ww so changes to it trigger wwstage rebuild.

lib/time is now in the toolchain transitive chain via lib/os. No
selfhost cmd calls time.add/time.diff today; bootstrap is safe.
Latent risk: any future selfhost edit adding time.add/time.diff
would surface task #15 (nested-if label-counter skew in lib/time/
add) as a bootstrap regression. File a fix-#15 before such an edit.

Bootstrap byte-id: ww2 == ww3 == ww4 for all five wwstage tools.

Makefile

@@ -113,7 +113,7 @@ $(BIN)/wwdump_ww: selfhost/cmd/wwdump/main.ww \
 		selfhost/cmd/wcc/cgen.ww selfhost/cmd/wcc/cgenexpr.ww \
 		selfhost/cmd/wcc/cgenstmt.ww selfhost/cmd/wcc/cgenutil.ww \
 		selfhost/cmd/wcc/cgendecl.ww \
-		lib/os/os.ww lib/strconv/strconv.ww lib/strings/strings.ww lib/ascii/ascii.ww lib/io/io.ww lib/memio/memio.ww lib/fmt/fmt.ww \
+		lib/os/os.ww lib/time/time.ww lib/strconv/strconv.ww lib/strings/strings.ww lib/ascii/ascii.ww lib/io/io.ww lib/memio/memio.ww lib/fmt/fmt.ww \
 		$(BIN)/ww $(BIN)/w6c $(BIN)/w6a $(BIN)/w6l \
 		$(LIB)/libwwrt.a | $(BIN)
 	cd $(BIN) && ./ww build \
@@ -134,7 +134,7 @@ $(BIN)/w6c_ww: selfhost/cmd/w6c/main.ww \
 		selfhost/cmd/wcc/cgen.ww selfhost/cmd/wcc/cgenexpr.ww \
 		selfhost/cmd/wcc/cgenstmt.ww selfhost/cmd/wcc/cgenutil.ww \
 		selfhost/cmd/wcc/cgendecl.ww \
-		lib/os/os.ww lib/strconv/strconv.ww lib/strings/strings.ww lib/ascii/ascii.ww lib/io/io.ww lib/memio/memio.ww lib/fmt/fmt.ww \
+		lib/os/os.ww lib/time/time.ww lib/strconv/strconv.ww lib/strings/strings.ww lib/ascii/ascii.ww lib/io/io.ww lib/memio/memio.ww lib/fmt/fmt.ww \
 		$(BIN)/ww $(BIN)/w6c $(BIN)/w6a $(BIN)/w6l \
 		$(LIB)/libwwrt.a | $(BIN)
 	cd $(BIN) && ./ww build \
@@ -152,7 +152,7 @@ $(BIN)/w6a_ww: selfhost/cmd/w6a/main.ww selfhost/cmd/w6a/types.ww \
 		selfhost/cmd/w6a/lex.ww selfhost/cmd/w6a/parse.ww \
 		selfhost/cmd/w6a/asm.ww selfhost/cmd/w6a/obj.ww \
 		selfhost/cmd/wcc/mem.ww \
-		lib/os/os.ww \
+		lib/os/os.ww lib/time/time.ww \
 		$(BIN)/ww $(BIN)/w6c $(BIN)/w6a $(BIN)/w6l \
 		$(LIB)/libwwrt.a | $(BIN)
 	cd $(BIN) && ./ww build \
@@ -168,7 +168,7 @@ $(BIN)/w6l_ww: selfhost/cmd/w6l/main.ww selfhost/cmd/w6l/sym.ww \
 		selfhost/cmd/w6l/obj.ww selfhost/cmd/w6l/dyn.ww \
 		selfhost/cmd/w6l/pass.ww selfhost/cmd/w6l/dynout.ww \
 		selfhost/cmd/w6l/out.ww selfhost/cmd/wcc/mem.ww \
-		lib/os/os.ww \
+		lib/os/os.ww lib/time/time.ww \
 		$(BIN)/ww $(BIN)/w6c $(BIN)/w6a $(BIN)/w6l \
 		$(LIB)/libwwrt.a | $(BIN)
 	cd $(BIN) && ./ww build \
@@ -181,7 +181,7 @@ $(BIN)/w6l_ww: selfhost/cmd/w6l/main.ww selfhost/cmd/w6l/sym.ww \
 # The driver pulls in lib/os (default search path) and selfhost/cmd/wcc
 # (for the bump arena). It then orchestrates w6c/w6a/w6l like the C driver.
 $(BIN)/ww_ww: selfhost/cmd/ww/main.ww selfhost/cmd/wcc/mem.ww lib/os/os.ww \
-		lib/strconv/strconv.ww \
+		lib/time/time.ww lib/strconv/strconv.ww \
 		$(BIN)/ww $(BIN)/w6c $(BIN)/w6a $(BIN)/w6l \
 		$(LIB)/libwwrt.a | $(BIN)
 	cd $(BIN) && ./ww build \