[idea] Double pane-dividers reduction

[MrDwarf7]

idea / problem

What

Fix doubled split-pane dividers and excess spacing required on row when pane_borders = true and pane_gaps = false.

• Two side-by-side (or stacked) panes currently each render the border on the shared edge, producing a doubled line (|| / =) instead of one.
• The bottommost pane renders a border (and reserves the row) on the edge that meets the terminal command line, stealing a row of pane real estate for no real reason.

Branch: mrdwarf7/fix-cull-outer-pane-borders

Implementation

• src/ui/panes.rs -- apply_pane_chrome border assignment now uses a single-owner scheme: each pane starts with Borders::ALL and drops RIGHT/BOTTOM when a neighbor sits on that side, so every shared divider is drawn exactly once (by the right/bottom pane). Under pane_gaps = false the bottom border is always dropped (internal horizontal dividers are owned by the lower pane's TOP; the bottommost edge meets the command line). (62 ins, 3 del)
• Added unit tests locking the behavior:
  • nested_split_without_gaps_uses_one_shared_divider_per_edge -- [L | TR/BR] split has one line per shared edge, no doubled borders, no bottom border on bottommost panes.
  • bottommost_pane_without_gaps_omits_bottom_border -- no pane keeps a BOTTOM border under pane_gaps = false.
• pane_gaps = true and pane_borders = false paths unchanged.

All 19 ui::panes tests pass; clean build.

---

Working on my fork (MrDwarf7/herdr) -- happy to PR.

requested change

Either the existing changes to be made the default or could potentially put them behind a flag (Something like compact_mode = true/false maybe?) - having atleast a setting to have the above changes be available in some way shape or form would be great.

why you want this

Why

A lot of wasted screen space on dense layouts. The doubled shared divider adds visual noise where a single line is more than enough, and the bottom border is mostly wasted space - there is no pane below it to divide from.

[markjaquith]

Hey, just calling out that some screenshots would be helpful in communicating the change!

[MrDwarf7]

Ahhhh yea, you're right hahah, oops. Have been working on a handful of things for Herdr and just wanted to start actually getting them up.
I'll get some before-and-after's later today to show off what the change actually does!