lessons.md: squash-merge can orphan follow-up/stacked commits

[WilfordGrimley]

Summary

• Adds one cross-session lesson to docs/lessons.md: a squash-merged PR discards the branch's original commits, so a follow-up commit added after review (PR Proposal H, item 3: flat vertical scroll + sheet-level virtualization #115 -> recovered as PR Item 3 follow-up: land the scroll/virtualization benchmark (recovers a commit orphaned by #115's merge) #178) or a PR stacked on the squashed branch (PR Proposal G, PR3: opaque-blob saved-decks API #88, lost when Proposal B: export-time per-side bleed normalization (core algorithm + real-render wiring) #66 squash-merged with --delete-branch) can be silently orphaned.
• Frames squash as the correct default for a flat, single-purpose PR; the caution is scoped to multi-commit/stacked situations, and the check is: verify the merged tree actually contains follow-up work rather than assuming the merge carried it.
• Based on current master (6656667, PR Apply GOOGLE_IMAGE harvest config: rate 8.0/s, concurrency 6 (task #165) #179's canary lesson already included) — pure append, no conflict.

Closes #181

Test plan

• python3 .github/scripts/docs_lint.py — clean
• pre-commit hooks (prettier, trailing-whitespace, end-of-file) — passed on commit