feat(editor): chart behaviour over time with uPlot

Draw the simulation's Stock trajectories with uPlot (ADR-0005): a real time
axis, hover-to-read values, and a playhead-marker hook the canvas animation
drives. SimChart owns the imperative uPlot lifecycle behind a small prop API.

docs/adr/0005-uplot-behaviour-chart.md

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+# uPlot draws the behaviour-over-time chart
+
+_Part of [meadows](../../README.md) · see [DESIGN.md](../../DESIGN.md)._
+
+The Results panel (ADR-0004's phase 2) traces each Stock's value over time. It
+began as a **hand-built SVG** — a deliberate lean-substrate choice, no charting
+dependency. But a polyline is all that substrate could draw: no time axis, no
+gridlines, and a legend that could only show a Stock's _final_ value, never the
+value under the cursor. For a tool whose whole point is reading _behaviour over
+time_, "what is this Stock at t = 40?" is the question you most want to answer by
+pointing at the curve.
+
+## Decision
+
+Adopt **uPlot** (`uplot`, ~15 KB gzip) — a small, fast canvas time-series
+library — for the behaviour-over-time chart. It earns the dependency with the
+three things the SVG could not give: a real **time axis** with ticks and
+gridlines, and a live legend that **reads each Stock's value at the hovered
+time**.
+
+uPlot is imperative and canvas-based, so it is wrapped in **`SimChart.vue`**,
+which owns its lifecycle (build on mount, `setData` on recompute, rebuild on a
+track-set change, `ResizeObserver` for width, destroy on unmount). `ResultsPanel`
+stays declarative: it assembles plain aligned data (Stocks only — the system's
+memory, per ADR-0004) and hands it down.
+
+## Considered Options
+
+- **Keep the hand-built SVG** (zero deps) — rejected: re-implementing axes,
+  tick placement, a hover cursor, and hit-testing _is_ rebuilding a charting
+  library, badly. The lean-substrate note was a code comment, not an ADR; nothing
+  prior is contradicted by spending one dependency where it clearly pays.
+- **`@gouvfr/dsfr-chart`** (the original prompt) — rejected: it requires the
+  entire DSFR design system (CSS + JS API) and embeds its _own_ Vue and Chart.js,
+  so we would ship Vue twice and a second design language clashing with DaisyUI.
+  It is built for French-government dashboards (region maps, gauges), not generic
+  time series.
+- **Chart.js / vue-chartjs** — viable and same engine class, but heavier; uPlot
+  is lighter and faster for many series and live-updating data, which is exactly
+  the simulation's shape.
+- **uPlot (chosen)** — lightest credible option that gives the axis + hover.
+
+## Consequences
+
+- This is the project's **first runtime charting dependency**. The price, as with
+  Vue Flow (ADR-0002), is a thin wrapper layer: `SimChart.vue` translates between
+  reactive props and uPlot's imperative API.
+- The x scale runs with **`time: false`** — x is _simulation_ time (`start…stop`),
+  not wall-clock, which uPlot would otherwise format as calendar dates.
+- Track colours are read from the **DaisyUI theme variables**. The app ships a
+  single `light` theme, so they are resolved once at build time; a future dark
+  theme would need the plot re-initialised on theme change.
+- Swapping charting libraries later means rewriting only `SimChart.vue` —
+  `ResultsPanel` passes plain `[times, ...tracks]` aligned data and knows nothing
+  of uPlot.