feat(dashboard): enable unified snapshot for atomic first paint

Default the dashboard snapshot flag to ON (disable with
NEXT_PUBLIC_DASHBOARD_SNAPSHOT=false). The legacy four-cell waterfall
let the mood cache resolve first and pop in ahead of the rest of the
strip; the snapshot path hydrates every tile from one un-gated cell so
the whole strip shares a single completion moment.

Defaulting on rather than gating on === "true" sidesteps the
NEXT_PUBLIC_* build-time-baking trap: the production image and dev/e2e
all exercise the snapshot path without a special build flag. Fully
reversible to the legacy path via the env var. Comment-only sync in
page.tsx; no logic change there.

src/app/page.tsx

@@ -120,8 +120,8 @@ function getHourForTimeZone(timeZone?: string): number {
  * v1.7.0 — first-paint gate for the dashboard tile strip.
  *
  * `primaryLoading` must come from whichever query actually drives the
- * tiles: the snapshot cell under `NEXT_PUBLIC_DASHBOARD_SNAPSHOT=true`,
- * the slim analytics cell otherwise. A disabled TanStack query reports
+ * tiles: the snapshot cell by default, the slim analytics cell when
+ * `NEXT_PUBLIC_DASHBOARD_SNAPSHOT=false`. A disabled TanStack query reports
  * `isLoading: false` (idle fetch status), so keying off the wrong
  * source flashes the empty state for the whole fetch. Pure + exported
  * so the gate has direct unit coverage without mounting the page.
@@ -245,12 +245,12 @@ export default function DashboardPage() {
   // Both queries share `caches.analytics` server-side so warm hits
   // stay free, and TanStack's parallel-mounting keeps the network fan-
   // out flat.
-  // v1.7.0 W6 — unified first-paint snapshot (reversible rollout flag).
-  // When `NEXT_PUBLIC_DASHBOARD_SNAPSHOT=true`, every tile hydrates from
-  // ONE un-gated `/api/dashboard/snapshot` cell so the whole strip
-  // shares one completion moment and the `/api/auth/me` round-trip
-  // leaves the cold critical path. Flag OFF keeps the legacy four
-  // independent cells (today's behaviour, byte-identical).
+  // Unified first-paint snapshot (reversible rollout flag, default ON).
+  // Every tile hydrates from ONE un-gated `/api/dashboard/snapshot` cell
+  // so the whole strip shares one completion moment and the
+  // `/api/auth/me` round-trip leaves the cold critical path. Set
+  // `NEXT_PUBLIC_DASHBOARD_SNAPSHOT=false` to fall back to the legacy
+  // four independent cells.
   const snapshotEnabled = isDashboardSnapshotEnabled();
   const snapshotQuery = useDashboardSnapshot(snapshotEnabled);
 

src/lib/dashboard/snapshot-flag.ts

@@ -1,18 +1,21 @@
 /**
- * v1.7.0 W6 — reversible rollout flag for the unified dashboard
- * snapshot consumption (R-firstpaint §6 rollout).
+ * Reversible rollout flag for the unified dashboard snapshot
+ * consumption.
  *
- * The `GET /api/dashboard/snapshot` endpoint and the
- * `insight-pregenerate` cron ship inert in step 1. This flag controls
- * step 2 — whether the dashboard page reads every tile from the single
- * snapshot cell (flag ON) or keeps the legacy four independent cells
- * (flag OFF, today's behaviour, zero risk). Default OFF so the swap is
- * opt-in until the stagger is verified gone in production.
+ * Controls whether the dashboard page reads every above-the-fold tile
+ * from the single `GET /api/dashboard/snapshot` cell (default) or falls
+ * back to the legacy four independent cells (slim analytics + thick
+ * analytics + mood + widget layout). The snapshot path is the desired
+ * behaviour: one un-gated request hydrates the whole strip so every
+ * tile shares one completion moment, instead of four parallel cells
+ * where the mood cache resolves fastest and pops in ahead of the rest.
  *
- * Build-time `NEXT_PUBLIC_*` env var so the bundle is statically
- * branched; flipping it is a redeploy, not a runtime toggle, which
- * keeps the off-path bundle identical to today's.
+ * Default ON. Set `NEXT_PUBLIC_DASHBOARD_SNAPSHOT=false` to fall back
+ * to the legacy multi-cell path. Defaulting on (rather than gating on
+ * `=== "true"`) avoids the `NEXT_PUBLIC_*` build-time-baking trap: the
+ * snapshot path is exercised in dev / e2e without a special build
+ * flag, and the production image gets it without a Dockerfile ARG.
  */
 export function isDashboardSnapshotEnabled(): boolean {
-  return process.env.NEXT_PUBLIC_DASHBOARD_SNAPSHOT === "true";
+  return process.env.NEXT_PUBLIC_DASHBOARD_SNAPSHOT !== "false";
 }