When the parent's disk crosses its floor, running children stop writing, new cells are refused, and the owner hears once #10873

Replaces #10852, which twice started before its base existed: it extends src/spawn/disk_floor.ts and the disk-floor vitals line that #10850's pull brings, so it waits for #10850 to land.

Done when:

• [x] Child forges already running go read-only when the parent's volume is below the floor: their forge refuses writes and says why in one plain line (or is stopped and kept asleep until room is made).
• [x] App and check cells inside the parent are refused below the floor, with the same reason.
• [x] The owner gets exactly one notification when the floor is crossed, none again until it is crossed anew.
• [x] A test for each.

Files: src/spawn/disk_floor.ts, src/spawn/cell.ts, src/kernel/vitals.ts, src/apps/process.ts

Reviewed pull #10988 at its head: ticked Child forges already running go read-only when the parent's volume is below the floor: their forge refuses writes and says why in one plain line (or is stopped and kept asleep until room is made).; App and check cells inside the parent are refused below the floor, with the same reason.; The owner gets exactly one notification when the floor is crossed, none again until it is crossed anew.; A test for each..