spike(rows-first): two-message state_change protocol behind env gate

[paddymul]

Draft / spike — not for merge. Validates the wire shape for "show rows first, compute stats after" without going async. Synchronous server, just message reordering + an IOLoop.call_later yield.

Sits on top of #785 (uses _defer_summary_sd + recompute_summary_sd hooks that this PR adds to the dataflow).

What it proves

• A single buckaroo_state_change produces two initial_state frames on the wire.
• An infinite_request fired by the client between the two frames is serviced and its parquet rows arrive before phase 2. The call_later(0.01) between phase 1 and phase 2 gives inbound socket reads a real time window to win the race.

Test: tests/unit/server/test_rows_first_spike.py (2 cases).

How to try it

Set the env flag and exercise normally:

BUCKAROO_ROWS_FIRST_SPIKE=1 uv run ...

Default (flag off) keeps today's single-frame rebroadcast. Existing test_load_expr tests pass unchanged.

Open questions

• xorq path coverage. XorqServerDataflow may not route through the _defer_summary_sd short-circuit on the base DataFlow. First spike test had to drop a "stats actually differ between phases" assertion because it was failing on xorq. This is the most important item to resolve before un-gating — without that assertion the spike test would still pass if recompute_summary_sd() were a no-op, so we can't detect regression of the spike's whole point.
• Inter-phase state_change race. Between phase 1 broadcast and the call_later firing, a second state_change can land. Phase 2 then computes stats against a processed_df that already reflects the second state, but the broadcast payload mixes phase-2 stats with what the user thinks is the first interaction. The spike doesn't try to address this; needs a serialise / cancel-previous strategy before un-gating.
• Delay tuning. 10ms is a localhost-only number — loopback WS RTT is well under 1ms. For remote deployments (RTT 30-100ms+) 10ms is shorter than the round-trip and the spike's premise breaks. Real prod tuning must measure end-to-end and pick a value that reliably loses the race to a network-bound infinite_request.
• Cache-hit fast path. When filt_sd_key for the new state is already in summary_stats_cache (search-box backspace, oscillation), there's no compute to defer — phase 2 would be near-instant. The spike doesn't short-circuit and emits two frames anyway, which is wasteful.
• Phase-2 message type. Both phases currently send type: "initial_state". JS will re-apply full state twice. Worth deciding whether phase 2 should be a distinct type (e.g. stats_update) so the front-end can target a cheaper re-render of just the stats-driven UI, rather than re-running the full initial_state handler.
• No before/after timing. PR currently asserts the protocol works but doesn't measure whether it produces a perceptible win on a realistic dataset. One A/B measurement (rows-rendered latency, with/without flag, on a search that triggers a real compute) would justify or kill the protocol.

Stack

• feat(pinned-rows): ?key prefix marks an optional pinned row #777 ✅ merged (JS ?key pinned-rows)
• feat(sd-cache): keyed summary-stats cache (raw / clean / filt scopes) #783 ✅ merged (keyed SD cache)
• feat(scoped-sd): merge raw + filt scope SDs into prefixed merged_sd #785 ⏳ open (merge raw + filt scope SDs into prefixed merged_sd)
• This ⏳ draft (rows-first two-message protocol; depends on feat(scoped-sd): merge raw + filt scope SDs into prefixed merged_sd #785)

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