Dashboard-level chart palette

A dashboard names the data colours its charts draw with: an ordered, distinct
subset of the `--chart-1…5` ramp, stored on the settings channel that already
carries `theme` and `locked`. No backend or client change — `SettingDef.value`
is free-form and a name this build does not wire up is left alone rather than
refused.

Naming none is the whole ramp, which resolves to the identical token per
series as the `--chart-${(index % 5) + 1}` charts drew with before, so every
existing dashboard is unaffected. `palette.check.ts` asserts that equivalence
rather than trusting it.

Distinct slots, not free assignment with repeats: only slot 1 against slot 5
clears 3:1 within the ramp, so two traces on one slot could not be told apart.
Status colour is deliberately outside the palette — a fault is `--destructive`
because of what it means, not because of where it sits.

The provider is mounted by the editor as well as the view, so picking a
palette repaints the charts beside the panel instead of describing them.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016ZeGnqVsf5VHQqvz4HdUhN

scripts/seed_demo.py

@@ -1234,7 +1234,11 @@ def seed_dashboard(api: Api) -> None:
                     "value": "system",
                     "message": msg(HOUSE, "panel_theme"),
                     "dtype": "str",
-                }
+                },
+                # The ends and the middle of the ramp rather than its first
+                # three steps: the Power chart draws three lines, and slots
+                # 1/3/5 stand further apart than 1/2/3 do.
+                "palette": {"value": ["1", "3", "5"]},
             },
             "pages": PAGES,
         },