Edge: stop storing the push head and backup flag, 3 → 2 rows (stage 2 of 2)

[wingleeio]

Stacked on #746. Merge only after #746 has been live long enough that rolling back past it is off the table.

What

#746 derives ChatRoom/RegistryRoom heads from the log (MAX(seq) floored by seqFloor / gcFloor, with the stored copy as a lower bound). It gates the nightly backup on head > backupSeq. It still stores headSeq / seq and backupDirty on every push, because older releases read only those.

This PR stops storing them. A push then writes only its data: 3 → 2 rows, about $7 → $5 a day at current traffic.

Rollback

The floor is #746. Releases older than that trust only the stored head, which no longer keeps up here, so they would reissue seqs. #746's readers handle state written by this release, and the tests pin that:

• a ChatRoom head the stored copy never saw continues at the right seq;
• a RegistryRoom seq the stored copy never saw continues without reissuing;
• backups run with no dirty flag at all.

Tests

• Write counts (workerd cursors): append 2, ChatRoom push 2, RegistryRoom one-row push 2.
• tsc ✅
• vitest run (63) ✅
• vitest run -c vitest.workerd.config.ts (34) ✅
• wrangler deploy --dry-run ✅

🤖 Generated with Claude Code

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