No check or build cell starts while the forge's disk is 80% full #11799

Part of #11686, item 6 ("a hard guard that pauses releases and builds at 80%"). Releases already pause at 80% (RELEASE_DISK_USED_MAX in src/keeper/cadence.ts, landed with #11686's pull).

Done:

• [x] A check's or build's step cell (runStep in src/apps/process.ts) does not start at 80% of the disk under the forge's data used; its log says the one line why, and the check is queued again, not failed red, when there is room (src/checks/disk_red.ts already treats a disk red as the machine's).
• [x] The 15% child-forge floor (src/spawn/disk_floor.ts) is unchanged.
• [x] Tests: a step at 81% used is refused with the line; at 79% it runs; a check refused this way is not counted against its pull.

Files: src/apps/process.ts, src/checks/disk_red.ts, test/disk-floor.test.ts

Reviewed pull #11808 at its head: ticked A check's or build's step cell (runStep in src/apps/process.ts) does not start at 80% of the disk under the forge's data used; its log says the one line why, and the check is queued again, not failed red, when there is room (src/checks/disk_red.ts already treats a disk red as the machine's).; The 15% child-forge floor (src/spawn/disk_floor.ts) is unchanged.; Tests: a step at 81% used is refused with the line; at 79% it runs; a check refused this way is not counted against its pull..