tests: harden parallel test execution

[yisding]

What changed

• cap -n auto at 8 workers and use per-test load scheduling in socket-free quick and coverage lanes
• run direct-fork tests in an explicit serial slice without xdist or watchdog helper threads
• avoid creating pytest-asyncio runners for synchronous tests while retaining async task leak checks
• consolidate five cold mypy consumer checks into one downstream type-contract test
• join a test-owned thread and update CI, documentation, validation selectors, and policy guards

Why

The suite mixed xdist workers, direct os.fork() calls, a global async autouse fixture on synchronous tests, and several repeated cold mypy processes. That combination created deadlock risk, noisy teardown behavior, uneven scheduling, and redundant work.

Impact

The quick lane remains broad while excluding process-global and xdist-unsafe tests. Static typing assertions are preserved in one dedicated CI step. The final local quick run completed in 2:44, compared with 3:51 before separating slow and serial work.

Validation

• quick xdist lane: 8,216 passed, 174 skipped
• serial direct-fork slice: 5 passed
• consolidated downstream type contracts: passed
• focused policy and docs guards: 93 passed
• full pre-commit suite: passed
• mypy for src/easycat: passed

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