Times without a time zone are UTC; Lighter account summaries; positions freshness

[0xFantomMenace]

Two fixes for 1.12.0: times without a time zone are UTC, and the account summary and positions freshness routes get wrappers.

Times without a time zone are UTC

• One helper, oxarchive/_time.py, replaces the per-resource conversion copies. An int passes through; an aware datetime is its instant; a naive datetime is UTC; a string of digits is Unix milliseconds; any other string is ISO 8601 (a trailing Z is accepted), UTC when it has no offset, and a date alone is midnight UTC.
• Before, a naive datetime and an offset-less ISO string were read as the machine's local time. Under TZ=America/New_York, "2026-09-01", "2026-09-01T00:00:00" and datetime(2026, 9, 1) all became 1788235200000 (04:00 UTC); they are now 1788220800000.
• A value that is not a timestamp raises ValueError instead of being dropped from the request.
• A naive datetime.now() is local wall-clock time, so the README and docstring examples now use datetime.now(timezone.utc).

New wrappers

• client.lighter.positions and client.rh_lighter.positions: account(account_index), account_history(account_index, start=, end=, cursor=, limit=), iterate_account_history() and async versions, for /accounts/{account_index}/account and /account/history. Rows are AccountSummary with the account's position aggregates.
• client.data_quality.positions_freshness() (and apositions_freshness()) for /v1/data-quality/positions, typed as PositionsFreshness.

Checks

• pytest: 173 passed, also under TZ=Asia/Kolkata and TZ=Pacific/Auckland; the new time zone tests set America/New_York themselves.
• ruff and mypy --strict: no new findings compared with master.
• The new methods were called against the production API and every response parsed.