fix(observe): an observer edit must not delete evidence they never saw (bd-2218)

[hyasin270]

What's wrong

The Flow prefills each Evidence box with the first PREFILL_TEXT_CAP characters of the AI draft. When the leader edits that box, applyObserverEdits writes the returned value straight over the original — deleting everything past the cap.

Nothing in the Flow signals that more text existed, so the one person who could catch it never sees it. The shortened text is what feeds the teacher's report.

Why the 300 → 600 change didn't close it

74abad00 raised the cap. That rescues the large majority of notes, but it moved the threshold, not the mechanism.

Measured against NIETE prod on 2026-07-21:

v1 evidence/improvement fields	338
over the old 300 cap	227 (67%)
already over the new 600 cap	9, across 5 of 13 sessions (longest 681, p99 654)
v2 coach-edited fields at exactly 300 chars	156

Text does not stop at exactly 300 characters 156 times by chance — that is the earlier cap already landing in the copy that feeds the report.

The fix

Compare against the full original, and keep the part the leader never saw:

const full   = String(orig[field] || '');
const shown  = full.slice(0, PREFILL_TEXT_CAP);   // all they ever saw
const unseen = full.slice(PREFILL_TEXT_CAP);      // '' unless the draft ran long
if (val !== shown && val !== full) textChanged += 1;
if (val !== shown) ind[field] = unseen ? `${val}${unseen}` : val;

The edit is honoured in full. An edit may shorten what was reviewed; it must not touch what wasn't.

Testing

Red-first, bot/tests/observe/observe-evidence-unseen-tail.test.js:

• editing a capped Evidence box keeps the text past the cap — fails on current main, passes here
• leaving a capped box untouched keeps the full original — guard, passes throughout
• a field shorter than the cap is replaced exactly, no tail bolted on — guard, passes throughout

Full tests/observe suite run with --runInBand. The only two failures (seed-field-officers, observe-subject-flags) fail identically on unmodified main — pre-existing, unrelated.

Note for the reviewer

Reported by Warda and Mubashar. They have been told on the card that the 600 cap does not cover every case, so this closes a promise already made to them.

Closes: bd-2218