test: make board heartbeat checks deterministic

[dgageot]

The three pkg/board heartbeat tests slept in real time (50ms, 250ms, 120ms)
to exercise the idle watchdog and cadence-reset logic, so they were slow and
prone to flaking under load. They also relied on a package-level var streamIdleTimeout that tests mutated directly, and drove the SSE stream over
a real Unix socket even when the behavior under test had nothing to do with
transport.

This converts TestStreamEventsIdleWatchdogAbortsSilentStream,
TestStreamEventsHeartbeatLinesResetWatchdog, and
TestStreamEventsNoHeartbeatNoWatchdog to Go 1.27's testing/synctest,
backed by httptest.NewTestServer instead of a live socket, so fake time
drives the heartbeat cadence and watchdog timeout instantly and
deterministically. streamIdleTimeout becomes a const now that no test
needs to shrink it, and the watchdog test asserts the exact 45-second timeout
boundary (streamIdleTimeout - time.Nanosecond still running,
streamIdleTimeout triggering errStreamIdle) instead of a shortened
stand-in value. The heartbeat-cadence test now waits out all six 15-second
pings, the production interval, and asserts the resulting 90-second elapsed
duration.

A new TestClientUnixSocket keeps a dedicated integration test that still
talks to the client over a real Unix socket, since none of the synctest
conversions above exercise that transport anymore.

No production behavior changes: streamIdleTimeout's value is unchanged, and
the watchdog/heartbeat semantics in pkg/board/client.go are untouched aside
from the var to const conversion.