tmpdir: write .origin sidecar to record session ownership

[jawauntb]

Refs #14935 (part 1 of 3).

What

Each pytest-of-<user>/pytest-N/ session directory now contains a .origin
file next to .lock, recording:

• rootpath: the absolute pytest rootpath
• version: the pytest version that created it
• pid: the PID of the creating process
• host: the hostname of the creating machine

Example:

$ cat /tmp/pytest-of-me/pytest-2/.origin
rootpath=/Users/me/proj-a
version=9.2.0
pid=48211
host=laptop.local

Why

Right now nothing in the session directory records which rootdir produced it.
External cleanup tooling (workstation janitors, CI cleanup steps, editor
temp sweeps, ad-hoc du-and-rm passes) has to reconstruct ownership at
delete time from live process state (/proc walks, lsof). Getting that
wrong deletes live scratch; getting it conservative leaves disks full.

cat .origin closes that gap without any change to how pytest itself manages
retention.

Scope

This PR only adds the sidecar. It is intentionally additive and does not
change any existing behavior:

• No public API changes to TempPathFactory, tmp_path, tmp_path_factory,
  or --basetemp.
• No cleanup logic changes.
• No retention or layout changes.
• The _rootpath field on TempPathFactory gets a default of None, so
  external callers constructing the factory directly (as several existing
  tests do) are unaffected.

Two follow-ups are drafted in #14935 and will land as separate PRs so each
can be reviewed on its own merits:

• Pid-liveness check in ensure_deletable (turns LOCK_TIMEOUT = 3 days
  into a fallback rather than the primary signal).
• Per-rootdir retention layout (tmp_path_layout = "per-rootdir"), which
  is where the sidecar starts to earn its keep — retention becomes scoped
  per project instead of shared across every rootdir a user has ever run
  pytest against.

Safety

Writing the sidecar is best-effort — any OSError during the write is
swallowed. A read-only mount, permissions error, or full disk cannot
break a test run. Covered by TestOriginSidecar::test_origin_write_failure_does_not_break_run.

Tests

New tests in testing/test_tmpdir.py::TestOriginSidecar:

• test_origin_written_next_to_basetemp — asserts the file exists with all
  four fields correctly populated.
• test_origin_written_when_basetemp_given — --basetemp also gets a
  sidecar so external tooling can attribute both cases uniformly.
• test_origin_write_failure_does_not_break_run — proves the swallow path.

Existing test_tmpdir.py tests (61 passed, 1 skipped locally) plus
test_pytester.py and test_config.py (340 passed, 1 skipped, 2 xfailed
locally) confirm no regressions.

Changelog

changelog/14935.improvement.rst added, keyed to the tracking issue.

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