docs: correct UPSTREAM_DIVERGENCE.md entries for pingdotgg#1996

- Split upstream pingdotgg#1996 into two ledger entries for the two MarCode PRs
  that port it: PR #69 (sidebar row timestamp behavioral fix, 524e93a)
  and PR #70 (store.ts shell-stream-authority correctness refactor,
  506b808).
- Remove "NOT yet equivalent" note in the Already-equivalent section now
  that both halves are ported.
- Drop pingdotgg#1996 from Pending real work; only pingdotgg#2246 remains.
- Add plan-file pointer (/Users/tyulyukov/.claude/plans/cached-napping-sundae.md)
  to the pingdotgg#2246 row so the next cycle's porter has the staged-commit strategy.

Corrects the earlier claim that pingdotgg#1996's store.ts refactor was "cleanup
with no behavioral delta" — it's a real correctness fix aligning MarCode
with the stream-separation contract MEMORY.md already requires.

Author: tyulyukov