Refresh build status line periodically

[Alizter]

The build duration in the status line currently advances only when another
scheduler event redraws the line. During long-running rules it can therefore
appear frozen.

This change installs a scheduler-owned 200 ms periodic timer in the async I/O
loop. Each tick sends a coalesced, lowest-priority status-line refresh event.
The event wakes an idle scheduler, while ordinary scheduler events subsume a
pending refresh. Rendering continues to run on the scheduler thread through
the existing status-line refresh hook.

The timer's lifetime is tied to the async I/O loop and it is stopped and
destroyed during scheduler shutdown. No console backend or progress renderer
changes are needed.

Tests:

• ./_boot/dune.exe runtest test/blackbox-tests/test-cases/status-line-refresh.t --force
• dune build bin/main.exe
• dune runtest test/blackbox-tests/test-cases/watching/rpc-build-status-line.t
• manual PTY smoke test with a blocked long-running rule

[rgrinberg]

Something is fishy in this PR

Why would it be necessary to cancel the timer that is updating the status line?

Why do we need the additional optimizations on top if we're updating the status five times a second?

[Alizter]

Something is fishy in this PR

Why would it be necessary to cancel the timer that is updating the status line?

You're right, it's completely unnecessary and was the cause of the performance issue I was trying to prematurely optimise. I've replaced it with something much simpler. I will iterate on it a bit further as I get the time.