Boards, step 5c: the forge samples every 15 s and registers the ring as the machine source #2345

Part of #1563.

Done looks like:

• [ ] A 15 s timer, started with the server and stopped on close, writes readMachine into the ring.
• [ ] The boards registry gains the machine source. An unreadable machine gives an empty series and a reason, never a crash.
• [ ] Test: the registry lists machine, and a query returns the points written.

Done on main already, waiting for you to close it: 910954a0cd5e2ed0de797496fd6fccdbcc849ccd 5ba342bb5e0e56e902510b313743b091beb4b313

Done on main in 910954a0 (#1576): src/boards/sampler.ts runs the 15 s timer into the ring, and src/boards/sources.ts registers the machine source. Nothing to re-plan. My token cannot close issues (403, scope), so a person needs to close this one. Reason: done.

Done on main already, waiting for you to close it: 910954a0cd5e2ed0de797496fd6fccdbcc849ccd 5ba342bb5e0e56e902510b313743b091beb4b313

State: deferred, waiting on #2502. It is done on main, and my token still cannot close it (403 again today). It keeps coming back as 'stalled' because stalled() in src/crew/held.ts skips the crew's done-on-main note. #2502 adds a done_on_main hold so a done issue is never handed back to a builder or the planner, and #2503 lets the planner close it itself. A person can close it now as done. Done in 910954a0: src/boards/sampler.ts, the machine source in src/boards/sources.ts.

Closed: already done on main in 910954a, 5ba342b.