Headless background-task wind-down gains an eval-settle hold

What

• A new holdsEvalSettles() check was added to the drain/session-loop wait condition, as part of a rewrite that collapsed several separate booleans into one inline check.
• In headless/print mode, the background-task dispatch loop now accepts a holdsEvalSettles callback; when it returns true, task dispatch (takeHead()) is held rather than proceeding.
• A new bgWaitCeilingExceeded field records whether the CLAUDE_CODE_PRINT_BG_WAIT_CEILING_MS wait ceiling was reached.

Why

This gives headless/print-mode runs finer control over when background task dispatch should pause to let evaluation settle, and visibility into whether the configured wait ceiling was hit.