feat(viz): add the display processor — value mappings and noValue #69

#30 landed units.ts (number → string) and colors.ts (value → colour). Neither handles the step before them: a field config can declare that 1 means "Up", that null renders as "No data", or that anything above 90 is "Critical" — and that substitution has to happen before the formatter ever sees the number.

Grafana calls this the display processor, and it is a single function because the order is not negotiable.

The contract

displayProcessor(config: FieldConfig, ctx: DisplayContext): (value: unknown) => DisplayValue

interface DisplayValue {
  text: string;      // what to render
  suffix?: string;   // kept separate, as units.ts does
  color: string;     // resolved, never a bare token
  numeric: number;   // for sparklines and bar widths — NaN when unmappable
}

The order, which is the whole point

1. Value mappings first. A mapping that turns 1 into "Up" must win over the unit formatter, or the cell reads 1.0 B.
2. noValue when the value is null/undefined/NaN and no mapping claimed it. A mapping can legitimately map null to text, so this is not simply a null check at the top.
3. Units and decimals on whatever numeric value survives.
4. Colour last, from fieldColorFn — except that a mapping may carry its own colour, which overrides.

The model (src/lib/dashboard/model.ts) already validates MAPPING_TYPES: value, range, regex, special. Mappings are ordered and first match wins — not last, not most specific.

Traps

• regex mappings are deferred to feat(panel): add the table panel #38 in the model and must stay deferred here. A user-supplied regex run against every cell of a 200k-row frame is a ReDoS vector against the browser tab. Whatever lands needs a linear-time matcher or a hard timeout, and that is its own design conversation.
• special covers null, nan, null+nan, empty, true, false — and true/false must match the boolean, not the strings "true"/"false".
• Range mappings are inclusive at both ends and from/to may each be null, meaning unbounded.
• The processor is called once per cell. A 500×50 table is 25,000 calls per redraw. Build the mapping lookup once in the closure — a Map for exact values, an array only for ranges — not per call. colors.ts has the same shape: thresholdScale is built once, thresholdColorAt is the hot call.
• A mapping's result.color is a stored token, so it goes through resolveColor, not into the DOM directly.

Scope

src/lib/dashboard/display.ts + tests. Pure and isomorphic, same rules as units.ts, colors.ts and frame.ts — no DOM, no imports beyond the sibling modules.

Blocks the stat (#37), table (#40) and gauge panels, which all render a mapped value rather than a raw one.