[CoreyKaylor]

Fixes #18.

Problem

When Normal has no background (a transparent theme), highlights.lua fell back to #000000 for two separate jobs:

1. Painting. Context rows, gutters, splits, filler, the sign column and the panel/status strips painted solid black (or a tint of it) over the terminal.
2. Blending. Every derived colour (bands, change emphasis, split and status tints) was mixed against black, even on light terminals.

Fix

resolve_backgrounds splits that fallback into two values:

• Reference background (blending): ui.theme.background (hex or a colour name) → Normal.bg → a neutral #1E1E1E / #FFFFFF picked by 'background'.
• Painted background: NONE when Normal has no bg, otherwise the reference. ui.theme.transparent = true/false overrides detection.

Add/delete/change bands stay opaque. Transparent themes now work with no configuration; ui.theme.background only tunes the band blend to the terminal's exact colour.

Related changes:

• Hidden separators (right pane ↔ overview strip) were hidden by fg = bg, which leaves a visible line when transparent. Those windows now get a window-local fillchars with a blank vert, keeping the user's other fillchars.
• vim.opt.fillchars:append({ vert = "│" }) also reset the current window's local fillchars, so it's now vim.opt_global. The Kitty edge bridges read the glyph from the global value for the same reason.
• The README documents ui.theme.background and ui.theme.transparent.

Tests

Suite 14 (tests/specs/10_connector.lua) covers:

• a transparent Normal leaves every page and status group with no bg, while bands stay solid
• background changes the blend without making the panes opaque, and colour names are accepted
• transparent = false / true force each mode
• a light-terminal fallback gives light bands
• opaque themes are unchanged
• hidden_separator_fillchars

The full suite passes (nvim --headless -u tests/run.lua → OK).

Not verified visually: Kitty ribbon anti-aliasing over a transparent terminal background.