fix(compaction): cap snapcompact frame payloads (#3866)
Bound rebuilt snapcompact image payloads by a per-request base64 byte
budget so long sessions stop re-sending multi-megabyte standing image
archives on every provider request; auto-compaction falls back to
context-full summaries when snapcompact output is too large.
Resolved snapcompact.ts conflict against the main font-rendering refactor
by keeping both renderabilityProbeText and the frame-budget helpers.
Fixed historyBlocks to emit the omitted-frame notice before the kept
(newer) images, since the byte budget drops the oldest frames — keeping
reconstructed blocks oldest-to-newest (addresses Codex P2 review).
Fixes#3792
can1357
fc24a70f5e