feat(tui): shade cache-read meter bands instead of recoloring them

[edwin-zvs]

A model owned two colors: its series color for new work, and that color blended toward black for the volume its provider served from cache. Two shades of one hue is what the meter otherwise uses for nothing, and a recessed color only reads as recessed next to the brighter one — a column that is entirely cache-read just looked like a dim model.

Now each model owns exactly one color, the one on its legend dot, and the split is carried by fill: cache-read cells are ▒ in the model's own color, new work stays █.

The one cell that can't hold the split

A cell can only be shaded if one band owns it outright and fills it to the top — the eighth-blocks have no half-density counterpart, and a background can't carry a glyph. So the single cell where a model's two parts meet goes to whichever owns more of it, which is the rule the partial cell topping a column already followed.

The bands are flipped to cache-first (cache-read at the base, new work above) so that cell always lands on new work, where a solid block is the correct rendering anyway. The degradation costs nothing visible in practice.

Boundaries between different models are unaffected — they still differ in color, so they keep the fg-over-bg encoding.

Notes

• specs/0167-fleet-token-meter.md updated: the "must be a color rather than an attribute" reasoning is superseded, and the cache-first ordering is recorded as a rendering requirement rather than a stylistic choice.
• No artifact: same render surface as feat(tui): split each token-meter band into new and cache-read work #1068, where an artifact was declined.
• Code lives entirely in the construct binary (crates/cli).