T2: cursor-aware incremental storage reads (readRows since + readRowsSince)

[philcunliffe]

Implements task T2 of change set incremental-sink-reads (LLP 0039/0040/0042).

Extends the kernel storage read contract so the forward and blob sinks can read only rows added since their last durable export:

• Back-compatible opts.since on readRows (absent ⇒ byte-for-byte the existing full scan; every current caller untouched).
• New cursor-aware readRowsSince(tablePath, { since, columns }) that pairs each internal-stripped row with its after continuation token.
• scanRowsFromTable gains opts.since and applies the seq > since predicate as a yielded-row filter. It is deliberately NOT pushed into icebird's scan({ where }): icebird couples file/row-group pruning with a per-row match that drops nulls (null > since is false in both hyparquet and JS), which would silently skip the legacy null-seq rows the migration must preserve (LLP 0040 risk [codex] Add durable cache spool #1). The design (§2) names this yielded-row filter as the fallback; a future null-aware icebird filter can layer the file-skip optimization on top without changing the contract.
• null-seq = new: a row whose _hyp_ingest_seq is null/absent is always yielded, so the one-time upgrade is at worst a full re-export, never silent data loss. A table that never carried the seq column yields everything.
• after is a monotonic high-water of real seqs, so a null-seq row carries the prior watermark forward and progress never regresses even when the scan visits seqs out of order (interleaved sources; risk [codex] Add root tests and remove donor tree #3).
• Kernel-types decl updated: SinkContinuation, ReadRowsOptions, extended readRows, new readRowsSince.

Checks

• npm test — 1404 pass / 1 skip / 0 fail
• npm run typecheck — clean
• npm run lint — clean

New tests (test/core/sink-reads-since.test.js) cover back-compat, after-token monotonicity, no-new-rows ≈0, incremental new rows, the null-seq migration contract, pure-legacy (no seq column) tables, and invalid-token rejection.

Task-Id: T2

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