feat(maestro): fill the row for the session in the pane

The web list still said "open in the focused pane" with a 1px ring, the last
edge left in it, and the TUI said it with the ● glyph alone. Both now use a
background.

Web: `--accent-fill`, the same 30% recipe `--preview-tint` uses, so the two
tiers differ by hue rather than by strength. The row rules are also rewritten
as one ordered run of equal-specificity `:where()` selectors, which is what
`docs/SIDEBAR.md` claims the ladder is — `.is-open` moves in from the bottom
of the multi-view block, where source order had been deciding its precedence
by accident.

TUI: `palette.pane_row_bg()`, blended partway from the card surface to the
selection surface, so the sidebar reads unfilled / half filled / filled. It
borrows the selection surface's foregrounds, since those are the ones picked
for a filled row, and the cursor still wins on its own row. Blended rather
than shipped as a third `Surface` — that would be eight more colours in each
of seventeen presets — with palette tests asserting it stays a visible step
from both surfaces and stays legible in every preset.

docs/SIDEBAR.md

@@ -177,6 +177,38 @@ surface; a colour chosen for the card background is never painted onto the
 selection background. Contrast is a property of the palette (values are RGB)
 and is asserted in the palette's tests, not left to the terminal theme.
 
+## Which row is filled
+
+Both clients say every one of these with a **background fill**, never an edge.
+A row is usually several of them at once, so they form a ladder: the tier
+furthest down wins the paint.
+
+| Tier | Web | TUI |
+|---|---|---|
+| Alive | 6% accent wash | — (the rail carries it) |
+| Under the pointer or the keyboard walk | preview hue, light | selection surface, while the sidebar holds the keyboard |
+| Open in some pane, not the focused one | 6% accent wash (`.is-open`) | — (the `●` glyph carries it) |
+| Showing in the pane | `--accent-fill` | `pane_row_bg()` — partway from the card surface to the selection surface |
+| Previewing (`docs/PREVIEW.md`) | `--preview-tint` | — (no preview colour role yet) |
+
+The web ladder is written as one ordered run of equal-specificity rules in
+`styles.css`, every qualifier inside `:where()`, so the order they appear in
+*is* the precedence. Add a tier where it belongs in that run, never by piling
+on another class to out-weigh the rule above it.
+
+The TUI's two filled tiers both carry the **selection** surface's foregrounds,
+since those are the colours picked for a filled row. `pane_row_bg()` is
+blended from the two surfaces rather than shipped as a third one — a third
+surface would mean eight more colours hand-picked seventeen times — and the
+palette tests assert it stays a visible step from both. The cursor's fill wins
+on its own row, so a highlight parked on the session already in the pane is
+still visible.
+
+The one edge left in either list is the TUI's `▏` marker, which stands in for
+the cursor's fill while the pane holds the keyboard, and the web's ring on a
+highlighted **folder** row — a folder previews nothing, so it has no fill to
+step up to.
+
 The duration text inherits its state's colour; the exited and command
 fragments use the surface's `muted` role, as do the diff stat's `~`, `?` and
 `↑` fragments. Only `+` and `-` step outside the surface's muted role, and they