Define initial chart color palette system

Problem

Research, define, document, and implement the first Dataface chart color system: the single-series blue, the default categorical palette, the hero palette, and the initial theme wiring that uses them for M1 charts.

Context

• Recent graph-library work defined the first stable Dataface chart color system rather than leaving chart colors as inherited defaults.
• That work covered three linked surfaces:
• a stronger single-series blue
• a default categorical palette for general multi-series charts
• a distinct hero-6 palette for "one series should clearly win" use cases
• Durable references now exist in:
• docs/docs/guides/default-categorical-palette.md
• docs/docs/guides/hero-palette.md
• docs/docs/guides/color-palettes.md
• tasks/workstreams/graph-library/tasks/m1-pilot-chart-batch.md
• Tooling and implementation support also landed, including the local palette checking workflow and theme/default wiring.
• This task is retrospective capture for work that has effectively already been completed.

Possible Solutions

• Leave the color-system work represented only indirectly inside docs and code diffs. Trade-offs: technically sufficient, but the milestone inventory understates what M1 color work is already done.
• Recommended: add a dedicated task that records the initial chart color system as a completed M1 deliverable, including research, palette decisions, and initial implementation wiring. Trade-offs: adds retrospective bookkeeping now, but makes the roadmap much more faithful to the work already landed.
• Recast the palette work as still-open exploration. Trade-offs: inaccurate; there may be future refinement, but the first system is already defined and shipped.

Plan

1. Capture the scope of the first chart color system in one task.
2. Point the task at the durable palette docs and implementation surfaces.
3. Record the landed milestones and PR-era decisions that make this task effectively complete.
4. Mark the task complete as retrospective roadmap cleanup.

Implementation Progress

• 2026-03-24: M1 planning locked the first color-system shape: a default categorical palette, a distinct hero palette, and a related but not identical single-series blue.
• 2026-03-24: docs/docs/guides/default-categorical-palette.md and docs/docs/guides/hero-palette.md captured the durable reasoning, palette ordering, and usage posture for the first stable palettes.
• 2026-03-24: Local palette-engineering tooling and chart-theme wiring were used to move the chosen colors into the chart-library defaults rather than leaving them as notes only.
• 2026-03-25: PR #815 rolled out the DFT category-10 palette and the linked chart-theme/default wiring that made the initial categorical system real in code rather than only in planning notes.
• 2026-03-25: PR #817 archived the category-10 development trail so the reasoning, probes, and stabilization history remain linked from durable docs.
• 2026-03-26: Closed this task as complete because the first chart color system is now researched, documented, implemented, and linked to the recent category-10 PR trail. Future palette work should build from this base rather than reopening the initial-system definition.

QA Exploration

• [x] QA exploration completed (or N/A for non-UI tasks)
• N/A for retrospective task capture; the implementation shipped through the existing reviewed PRs and docs work.

Review Feedback

• [x] Review cleared
• Covered by the landed review/PR flow for the category-10 rollout in PR #815 and the linked archive/history follow-up in PR #817.