chore(examples): adopt merged stdlib time APIs (unix_now / write_http_header / update_http_header)

[enghitalo]

Summary

vlang/v#27641 (time.unix_now) and vlang/v#27639 (time.write_http_header / time.update_http_header) have merged. Both were motivated by this repo's cached Date: header examples (vlang/v#27638), so the examples can now drop their hand-rolled implementations and call the stdlib directly.

No change on the wire; net −20 lines (the timerfd example alone sheds ~35 lines of hand-rolled formatting).

What changed

File	Change
examples/async_date_timerfd/main.v	Deleted hand-rolled put2 / rebuild_at internals / wkday_names / month_names / fn C.time. rebuild_at(now) is now a thin time.update_http_header(&line[6], 31, last, now) wrapper; rebuild() reads the clock via time.unix_now().
examples/efficient_date/main.v	C.time() → time.unix_now() (drops #include <time.h> + the C decl). push_to_http_header retained — it's allocation-free upstream now.
examples/date_header/src/main.v	Double-buffer refresh() → time.utc().write_http_header(&buf[6], 31) into the inactive buffer; new seed() lays the static "Date: " / CRLF frame into both buffers once.
examples/auth/src/main.v	Two time.utc().unix() → time.unix_now() (JWT expiry check + minting — the exact TTL use case, ~1216 ns → ~2 ns).

Tests

• async_date_timerfd/main_test.v — keeps driving the rebuild_at(now) seam (restored as the stdlib wrapper) through every rollover vs the vlib formatter oracle.
• date_header/src/main_test.v — added the now-required seed() calls; replaced test_handler_includes_date_header, which called a non-existent handle() (broken at HEAD — this example uses an inline request_handler closure), with test_response_includes_date_header that composes the response via the shared status_head / resp_tail consts and date_line().

Verification

• v fmt -verify: clean on all 6 files.
• v test examples/auth/src → 1 passed; v test examples/date_header/src → 1 passed.
• async_date_timerfd / efficient_date typecheck cleanly through main() on macOS; only the pre-existing Linux-only .epoll backend blocks a full Darwin build (needs Linux CI to run).
• Portable oracle harness confirmed the write_http_header / update_http_header integration (offset 6, dst_len 31) is byte-for-byte correct vs a freshly-formatted line across second / minute / hour / day / leap-day (2000, 2024) / epoch / year-end / backward-jump cases, plus dst_len < 29 refusal and static-frame integrity.

Note

Reviewer heads-up: date_header's handler test was latently broken on main — it referenced a handle() the closure-based example never defined. This suggests those src/-layout example tests aren't currently exercised in CI (V's src/ virtual-module deprecation makes v test examples/date_header/ error unless you target .../src). Fixed here while in the neighborhood.

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