Add the SSXNG_SYSTEM_LOG=0 test seam: RuntimeLog.emit and the sslocal
unified-log destination skip their os.Logger writes when it is set,
while stderr lines and the event sink are unaffected. Test execution
no longer floods the unified log store (a full validate run previously
recorded 795 entries under the shared subsystem, 743 from xctest and
52 from test-spawned agents).
The switch is injected by the schemes' test actions only (Run/Profile
keep system logging so debugging from Xcode still shows it), and set
explicitly in the fixtures that spawn the agent/sslocal processes,
including the fixture that strips inherited SSXNG_* path seams. The
main scheme is now declared explicitly to keep the variable out of the
Launch/Profile actions. Document the seam in ADR-0045.
Qiu Yuzhou