fix(board): reliability fixes for SSE lifecycle, watchdog, transport, and rendering

[dgageot]

The board's SSE event stream had several reliability gaps that caused missed events, stalled connections, and occasional rendering glitches under real-world network conditions.

This change tightens the full SSE lifecycle: the watchdog timer now resets on every received line rather than only on data frames, so comment and keep-alive lines properly defer the timeout. Keep-alive probes are disabled on the Unix domain socket transport, which doesn't need them and was generating spurious traffic. The heartbeat goroutine is now drained before the function returns, preventing a subtle goroutine leak on reconnect. The title-truncation helper now trims both halves of a split title consistently. A defer replaces the manual unlock in SetSamplingWithToolsHandler, fixing the mutex lint warning. Regression tests cover the watchdog reset behaviour and the title truncation, and timing margins in the heartbeat test were widened to reduce flakiness on slower CI runners.

All existing tests pass (task test), the linter is clean (task lint), and the binary builds (task build). The watchdog and heartbeat paths were also verified with targeted -race runs.

[docker-agent]

Assessment: 🟢 APPROVE

The changes are well-reasoned and correctly address the stated reliability gaps in the SSE lifecycle. Here is a brief summary of the review findings:

• Watchdog timer reset (client.go): Moving resetWatchdog() before the heartbeat-comment branch so every received SSE line (including : ping comments) resets the watchdog is logically correct and matches the advertised fix.
• DisableKeepAlives: true on Unix socket transport (client.go): Correct — keep-alive probes are meaningless on Unix domain sockets and could generate spurious traffic; disabling them is appropriate.
• Heartbeat goroutine drain (server.go): The heartbeatDone channel approach correctly prevents the goroutine leak on reconnect; the deferred closure stops the heartbeat and waits for heartbeatDone, and the goroutine closes it on exit.
• splitTitle truncation (tui/view.go): Truncating line1 symmetrically with the remainder is the correct fix for the rendering inconsistency.
• defer c.mu.Unlock() (session_client.go): Replacing manual Unlock() with defer is safer (panic-proof) and fixes the mutex lint warning as intended.
• Tests: Timing margin widening and new regression tests for watchdog reset and title truncation are appropriate.

No bugs were found in the newly introduced code (+ lines). All changes appear correct and improve reliability.