docs: describe the diff minimality budget by its cell product

Author: ThomasK33

docs/USAGE.md

@@ -218,7 +218,7 @@ agent-tty record diff <session-id> <session-id> --at-seq-a 0 --json
 - `--at-seq-a <seq>` / `--at-seq-b <seq>`: replay each side up to an Event Log sequence (default: latest). Diffing a session against itself at an earlier sequence shows how its screen evolved.
 - The JSON result carries `identical`, per-side `sessionId`/`capturedAtSeq`/`cols`/`rows`/`screenHash`, and a `diff` array of `{ op: equal | delete | add, text, aRow?, bRow? }` entries over the visible screen lines (0-based rows, no trimming or normalization — the same canonical lines that `screenHash` hashes). A side with an empty event log reports the pre-event blank screen with `capturedAtSeq: -1` (no event was replayed).
 - Human output is a unified-style diff with `---`/`+++` headers naming each side's session, sequence, and hash prefix.
-- The line diff is LCS-minimal for any realistic screen. As a bounded-memory safeguard, when the differing region (after matching the common prefix and suffix) exceeds roughly 2,000 lines on **both** sides, that region degrades to a plain delete-then-add block instead of a minimal diff — `identical`, both `screenHash` values, and full-screen reconstruction remain exact; only diff minimality is reduced.
+- The line diff is LCS-minimal for any realistic screen. As a bounded-memory safeguard, when the differing region (after matching the common prefix and suffix) is so large that the product of its two side lengths exceeds a 4,000,000-cell budget (for example 1,000 × 4,001 lines), that region degrades to a plain delete-then-add block instead of a minimal diff — `identical`, both `screenHash` values, and full-screen reconstruction remain exact; only diff minimality is reduced.
 - The command works entirely offline from `events.jsonl`; sessions may be running or exited. The exit code is `0` whether or not the screens differ — automation should read `identical` from the JSON result.
 
 ## Isolation