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.

test/wcc/900_stdlib.c

@@ -16,7 +16,6 @@ static const char *modules[] = {
 	"lib/strings/strings.ww",
 	"lib/io/io.ww",
 	"lib/errors/errors.ww",
-	"lib/os/os.ww",
 	"lib/strconv/strconv.ww",
 	"lib/sort/sort.ww",
 	"lib/path/path.ww",
@@ -33,13 +32,15 @@ static const char *modules[] = {
 	"lib/math/random/random.ww",
 	"lib/time/time.ww",
 	"lib/c/libc/libc.ww",
-	/* lib/bufio/bufio.ww and lib/fmt/fmt.ww moved off this list:
-	 * both graduated to io.stream-based sinks, which carry cross-
-	 * module type refs (*io.stream, io.closed, io.eof) that only
-	 * resolve once the driver concatenates `use`d modules. Coverage
-	 * lives at lib/bufio/bufiotest.ww + lib/fmt/fmttest.ww (wired
-	 * at 998_bufio_run.c and 970_fmt_run.c), plus the bufio.scanline
-	 * and fmt.println e2e rows in test/wcc/700_e2e.c. */
+	/* lib/bufio/bufio.ww, lib/fmt/fmt.ww, and lib/os/os.ww moved
+	 * off this list: each has cross-module type refs that only
+	 * resolve once the driver concatenates `use`d modules. bufio /
+	 * fmt graduated to io.stream-based sinks (*io.stream, io.closed,
+	 * io.eof); lib/os carries time.instant in filestat post-Commit B.
+	 * Coverage lives at lib/bufio/bufiotest.ww + lib/fmt/fmttest.ww +
+	 * lib/os/stattest.ww (wired at 998_bufio_run.c, 970_fmt_run.c,
+	 * 976_stat_run.c), plus the bufio.scanline / fmt.println e2e rows
+	 * in test/wcc/700_e2e.c. */
 	"lib/net/net.ww",
 	NULL
 };