Harden systest log filenames against concurrent-writer collisions

[mkarlesky]

Summary

Prep work for a future CI system-test sharding effort (deferred, not part of this PR): closes a real latent collision risk found while investigating that sharding design.

spec/support/system/spec_system_helper.rb's after(:each) hook writes each system test's result log to systests/systest.<pass|fail>.<test_name>.<timestamp>.log, where <timestamp> was second-precision with no PID. Two RSpec processes writing a log for identically-described tests within the same UTC second would silently overwrite one another. SystemContext's own per-test project directories already use Dir.mktmpdir and are unaffected — this was the one non-collision-proof path in the harness.

Fix mirrors the pattern this codebase already uses, proven safe, for SimpleCov's per-child-process resultset directories (spec/support/system/simplecov_boot.rb): add Process.pid and switch to microsecond-precision timestamps.

Test plan

• Spot check: launched two RSpec processes concurrently against the same checkout, both running the identical test with CEEDLING_SYSTEM_TEST_KEEP=all — before the fix this would collide; confirmed two distinct log files are now produced instead of one clobbering the other.
• Single-file diff, no behavior change to log content or pass/fail logic.
• Full CI suite (backstop for full-suite regression — this change is scoped to log-filename construction only, not exercised meaningfully by a partial local run).

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