spec: add burndown-sprint specification

plots/burndown-sprint/specification.md

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+# burndown-sprint: Agile Sprint Burndown Chart
+
+## Description
+
+A sprint burndown chart tracks the amount of remaining work in an agile sprint over time, plotted as a step (or line) series that descends as tasks are completed. A straight ideal-burndown guideline runs diagonally from the total committed scope on day one down to zero on the final day, giving the team an at-a-glance reference for whether they are ahead of or behind schedule. It is one of the most widely used project-management visualizations in software development, surfacing scope creep, stalled progress, and end-of-sprint risk early enough to act on.
+
+## Applications
+
+- Daily Scrum tracking of remaining story points across a two-week sprint to forecast on-time completion
+- Detecting scope changes mid-sprint, where the remaining-work line jumps up when new work is added
+- Comparing actual progress against the ideal trajectory to flag a team that is consistently behind
+- Release or milestone burndown over a longer horizon to communicate delivery confidence to stakeholders
+
+## Data
+
+- `day` (date) - calendar day of the sprint, from start date to end date
+- `remaining` (numeric) - remaining work at the end of that day, in story points or task count
+- `ideal` (numeric) - ideal remaining work for that day on the straight line from total scope to zero
+- `scope_change` (numeric, optional) - signed change in total scope on that day, for scope-change markers (positive = work added)
+- Size: 5-30 days (a typical sprint is 10 working days), single team
+- Example: a 10-working-day sprint starting with 40 story points, burning down to 0, with a +8 point scope addition on day 4
+
+## Notes
+
+- Render the actual remaining work as a step series (work is completed in discrete chunks), or a straight line if the library makes steps awkward
+- Draw the ideal burndown as a straight reference line from (start, total scope) to (end, 0), visually distinct (e.g. dashed, muted color)
+- Y-axis starts at 0 and represents remaining work (story points or tasks); X-axis is the sprint timeline
+- Shade weekends (non-working days) with a light background band to explain flat segments in the actual line
+- Mark scope changes with a vertical line, arrow, or annotation where total scope shifts mid-sprint
+- An optional burnup variant plots completed work rising toward a (possibly changing) total-scope line instead of remaining work falling to zero; keep the static, single-snapshot framing
+- Include a legend distinguishing the actual series from the ideal guideline
+- Below the ideal line is ahead of schedule; above it is behind — keep that reading obvious through color or annotation

plots/burndown-sprint/specification.yaml

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+# Specification-level metadata for burndown-sprint
+# Auto-synced to PostgreSQL on push to main
+
+spec_id: burndown-sprint
+title: Agile Sprint Burndown Chart
+
+# Specification tracking
+created: "2026-06-10T23:00:07Z"
+updated: null
+issue: 8654
+suggested: MarkusNeusinger
+
+# Classification tags (applies to all library implementations)
+# See docs/reference/tagging-system.md for detailed guidelines
+tags:
+  plot_type:
+    - line
+    - step
+  data_type:
+    - timeseries
+    - numeric
+  domain:
+    - project-management
+    - technology
+    - business
+  features:
+    - temporal
+    - comparison
+    - annotated
+    - trend