fix(operate): make Start say which of four things it will do

[pskeshu]

Audit findings 4 and 5.

The button

startRun branches on _mode into four different verbs. The button said "Start" for all four, and the mode selector lives in a block above it — so an operator who chose a mode and then looked away had nothing on the button to read.

mode	label	what it does
single	Acquire one volume	takes one volume and finishes
adaptive	Start timelapse
library	Run tactic
agent	Brief the agent

single is the one that matters — calling it "Start" invited an operator to believe they had started an experiment. The markup ships single's verb so there's no flash of the wrong one on load, and the label is restored after a run rather than reverting to "Start".

The fallback that was exactly backwards

const subs = _embryos.filter(e => e.role !== 'calibration').map(e => e.id);
return subs.length ? subs : _embryos.map(e => e.id);

Meant to be kind to a roster with no roles assigned. It could never do that: an embryo with no role, or 'test', or 'unassigned' already passes the filter — so subs is empty in exactly one case, when every embryo is marked calibration.

So it fired only when the operator had said "these are all references", and answered by imaging all of them as subjects. The precise opposite of the instruction, silently.

Gone. haveSubjects() refuses at the single point both roster-driven modes pass through, and distinguishes the two states — "no embryos" and "no subjects among your embryos" need different fixes.

Checks

Four source assertions: every mode has a verb (a mode added to setMode but not RUN_VERB silently falls back to "Start"), single's label contains no "start", subjectIds keeps no fallback, and the guard names both states.

Verified in the running app — labels track the mode with no flash, and all three roster states answer correctly: every-reference refused by name, empty roster refused differently, one real subject allowed through.