feat(cloud): cloud status reports the seen-txn ledger size per store (T13317)

[kryptobaseddev]

Task: T13317

Stacked on #1970 (feat/T13315-typed-skip).

Why

Team-lead's T13256 decision (option 3): seen-txn pruning stays off, as #1959 left it, until a proven refusal floor exists (T13318). Until then, the ledger's growth is made visible.

cleo doctor sync doesn't exist as a command. The store's sync report, including the undo budget the spec assigns to "doctor sync", is cleo cloud status's sync block, so the report goes there.

Change

• contracts: a new CloudStatusSeenTxns {rows, bytes, byStream} type. CloudStatusSyncStream.seenTxns is a required field.
• core: seenTxnReport(db) (store/sync/pull.ts) groups rows by stream. The byte estimate is the stored stream and txn text plus an 8-byte seq per row, with no page overhead. readStoreSyncStream fills seenTxns.
• CLI: the human sync clause adds <n> seen txn(s), about <size> KiB once the ledger has rows.
• No code path prunes the ledger.

Verification

• Tests:
  • cloud-sync-status: per-stream counts and the exact byte estimate across two streams; a second read sees the same rows (a read never prunes); the flags-off case reports zeroes.
  • nexus-cloud-cli-sync: the clause shows KiB with one decimal under 10 KiB and rounded above, and nothing while the ledger is empty.
  • Together with pull, 18/18 pass.
• Mutations: 2/2 killed (seq bytes set to 0; clause hidden).
• Typecheck: tsc --noEmit is clean for contracts, core and cleo.
• Biome: clean.
• Gates: 28, 38 (--base), 40, 4, 10, 32 (--base) and 6 (--check, as CI runs it) pass; lint-changesets passes.

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