Make the sandbox name pattern per-supervisor, not a mutable global

[seanwevans]

The problem

Supervisor.spawn declared global NAME_PATTERN, read it, and reset it to DEFAULT_NAME_PATTERN on every successful spawn:

def spawn(self, name, ...):
    global NAME_PATTERN
    ...
    pattern = NAME_PATTERN
    ...
    NAME_PATTERN = DEFAULT_NAME_PATTERN   # at the end of every spawn

Two problems:

1. A caller who installed a custom pattern silently lost it after one sandbox, and two threads spawning concurrently raced over the value — one thread's reset could reject the other thread's legitimate name.
2. The reset existed only to serve a test. tests/test_metrics.py rebound the module global and never restored it, relying on spawn to clean up. Production code was carrying a test affordance.

The change

Supervisor(name_pattern=...) — the rule belongs to the instance that enforces it, exposed as a .name_pattern property. When not given, the supervisor falls back to the module-level NAME_PATTERN at spawn time, so the documented global override still works for the process-wide supervisor. spawn no longer writes to it.

Restoration moves to where it belongs: the metrics test uses monkeypatch.setattr, which undoes itself.

Tests

Three new in tests/test_supervisor.py: a custom pattern is honored and rejects non-matching names; two supervisors keep independent patterns; and the regression itself — a custom global survives a spawn and is still honored on the next one.

Full suite 508 passed / 6 skipped (was 505). flake8 clean.

Note

I let black touch only the files this change edits. A newer black than the repo's pinned 23.9.1 wanted to reformat nogil.py, test_ebpf_contract.py, and test_thread_extra.py — unrelated, so I reverted those.

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