Reporting layer v1: scatter, dual-axis overlay, brush-zoom (#1606)

[erikdarlingdata]

The charting item of #1606, re-prioritized: "investment in reporting layer is crucial."

What's new

• Second measure (overlay) — a panel can carry one more same-source measure. It compiles as a second select expression (AS value2) over the same fact rows — the ratio two-operand precedent generalized: never a join, never a parameter, one query. Same aggregate/unit rulebook as the primary (extracted into a shared helper so the two can't drift); coherence-gated to scatter and ungrouped line/area, rejected everywhere else by name.
• Scatter — ranked two-measure viz: one point per group, x ranks by the primary, y is the overlay. Hover titles, drill-to-filter clicks, dual nice-scaled axes with unit captions.
• Dual-axis line/area — an ungrouped time series draws the overlay against its own right-hand axis (own scale/caption, reserved dashed color). The right gutter is reserved only when an overlay is present — existing charts render byte-for-byte unchanged.
• Brush-zoom — pointer-capture drag on any time-series chart re-RUNS the panel on the brushed absolute window: the auto bucket re-resolves, retention tiers re-route, and the Darling compose: CAGG routing has no rollup-availability gate — old-window custom-view panels throw 42P01 on plain-PostgreSQL stores #1665 partial-window notice stays honest. Zoom chip (with reset) survives empty/error results — a historical zoom can legitimately return one daily bucket.
• Absolute run windows — /api/compose/run (and therefore MCP run_custom_view_panel) gains windowStart/windowEnd: ISO-8601, precedence over hours, both-or-neither, start<end, 90-day ceiling, reject-not-clamp. Historical analysis becomes first-class.
• NowUtc decoupling — ComposeRunContext.NowUtc separates "when is now" from "when does the window end": routing and the retention notice measure age from actual now. Pinned: a 30→25-days-ago window routes to the daily rollup.
• CAGG AND-gate — one route serves both value expressions; if either measure can't remap to the rollup columns the whole panel reads raw.
• Composer UI: Second-measure picker, scatter in the chart list, all five viz-coherence mirrors updated, and stored definitions round-trip the overlay through edit-and-save. MCP catalog/tool prose updated (also fixes its stale stacked-bar omission).

Design + review

Plan reviewed adversarially by a separate tech-lead agent before implementation; all findings adopted (overlay coherence outside ValidateVizMode's table early-return, editor mirror threading, reject-not-clamp window semantics, zoom-chip-on-empty, param-order invariance pin, pivotTimeSeries value2 wiring, otherColumns exclusion).

Testing

• 9-case overlay rejection matrix + accept pins (scatter, ratio overlay with fixed aggregation).
• Compile pins: AS value2, parameter-count invariance with/without overlay, scatter SQL shape (ORDER BY value DESC + bound LIMIT), both-remap-or-raw on the rollup route, historical-window NowUtc routing.
• 4-case absolute-window rejection matrix (no live store needed — rejects before any round trip).
• Full suite 3140/0; live-gated against a fresh plain PostgreSQL 17: 3268/0.
• Post-merge: DARLING01 deploy + demo panels (duration-vs-executions scatter, CPU%+batches dual-axis) for the visual pass.

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