style(tui): restore Australis, $background → pure black (v0.6.3)

Reverts v0.6.2's over-correction. The operator clarified: the
complaint was specifically about the APP BACKGROUND going from
black to a shade of blue, not about the cumulative cast across
all Australis dark surfaces. v0.6.2 globally neutralized Ice + Sea
darks → too far.

## v0.6.3 = v0.6.1 palette + $background override only

Restored verbatim from v0.6.1:

  $foreground       #a9bcc3 (Ice white)
  $surface          #373b46 (Sea bright-black, chrome bg)
  $panel            #414751 (Sea dark 30, borders)
  $au-dark-30..60   Australis Sea palette
  $au-bright-70/80  Australis Sea brights
  $au-bright-white  #cce7ec (Ice highlight)
  Aurora accents    bright-blue/cyan/green — verbatim
  Dawn accents      red/yellow — verbatim
  _AU_DEMOTED       #86929d (Sea dark 60)
  _AU_DEMOTED_FAINT #6e7882 (Sea dark 50)

ONE deviation from Australis spec:

  $background  #222531 (Ice black) → #000000 (pure black)

Rationale: Ice black is RGB(34, 37, 49) — blue +44% over red. At
App-wide scale (the dominant fill across the entire screen) the
cumulative cast reads as "the app is blue" even though no single
rect is in the conventional-blue range. Other dark surfaces are
smaller chrome bands where the cool lean reads as character not
background; only $background gets the override.

## What stayed Australis

Every cosmetic element where the operator hasn't pushed back:
identity widget (Aurora bright-blue), pane-name (Aurora bright-cyan),
[done]/[error]/[cancelled] labels (Aurora green / Dawn red/yellow),
focus borders (Aurora blue / accent cyan), demoted telemetry text
(Sea dark-60), placeholder lines (Sea dark-50), Header/Footer chrome
(Sea bright-black bg + Ice white-blue fg), separators (Sea dark-30).

Brand fidelity preserved; only the dominant background surface
neutralized.

## Tests + smoke

241/241 GREEN; ruff clean. Live screenshot export:
- $background = #000000 (230 fill rects — dominant surface)
- $surface = #373b46 (29 fill rects — Australis Sea bright-black)
- Sea panels + dark-50 still present in chrome
- Aurora #6388D8 still primary

Patch bump (v0.6.2 → v0.6.3) — cosmetic refinement; no public-API
change.

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "hatchling.build"
 
 [project]
 name = "ratatoskr"
-version = "0.6.2"
+version = "0.6.3"
 description = "Worldtree Conversation API debug TUI — multi-pane observability dashboard"
 readme = "README.md"
 requires-python = ">=3.12"