feat: start time travel at inbound request entry via x-td-root-timestamp header

[jy-tan]

Summary

Enable deterministic time travel at the very start of an inbound replay request by reading a new x-td-root-timestamp HTTP header sent by the CLI. This fixes replay deviations caused by time-sensitive code executing before the first outbound span is matched - most notably django-cachalot, which incorporates timezone.now() into Redis cache keys.

Previously, time travel was only activated per-outbound-span (when a mock was matched). This meant any time-dependent logic that ran before the first outbound call (e.g., cache key generation, session expiry checks) would use wall-clock time instead of the recorded time, causing cache key mismatches and divergent code paths during replay.

Changes

• drift/instrumentation/django/middleware.py: Read HTTP_X_TD_ROOT_TIMESTAMP from Django's META headers and call start_time_travel() immediately in _handle_replay_request, before any downstream middleware or view code runs.
• drift/instrumentation/wsgi/handler.py: Read x-td-root-timestamp from WSGI environ headers and call start_time_travel() at the start of _handle_replay_request.
• drift/instrumentation/fastapi/instrumentation.py: Read x-td-root-timestamp from ASGI headers and call start_time_travel() at the start of _handle_replay_request.

Design notes

• The header is optional and best-effort; if the CLI does not send it (older CLI versions), time travel falls back to the existing per-outbound-span mechanism with no behavioral change.
• Django mangles headers into HTTP_X_TD_ROOT_TIMESTAMP format, so the Django middleware reads from http_x_td_root_timestamp in the lowercased headers dict, while WSGI/FastAPI use the standard x-td-root-timestamp.
• This is a companion change to the CLI-side PR that adds the x-td-root-timestamp header to replay requests.

[cubic-dev-ai]

No issues found across 3 files

[jy-tan]

Not needed anymore since we use root span timestamp for SendSetTimeTravel gRPC call on the CLI side. See Use-Tusk/tusk-drift-cli#154.