Panels: draw only some border sides

[ralyodio]

Closes #59.

A border was all four sides or nothing. Anything else meant a divider() plus
manual padding, which gives you no corner joins and no way to say "just a top
rule" — the thing you reach for building a header strip, a sidebar rail, or a
footer that should not look boxed in.

ui.panel({ title: "Header", border: "single", sides: ["top"] }, (p) => { ... });

sides takes "all" (the default), "none", or a list. The interior follows
the sides actually drawn, so a top-only panel costs one row rather than two:

┌────────┐        ──────────        │ ab     │        ┌─────────
│ ab     │         ab               │        │        │ ab
│        │                          │        │        │
└────────┘                          │        │        │
   all              ["top"]      ["left","right"]  ["top","left"]

The collapse work already had the vocabulary

As the issue guessed, borderGlyph/borderBits speak in edge bits, which is
exactly what this needs. A corner belongs to the two sides that meet there, so
it exists only when both are drawn; where one is, the rule runs straight
through the cell the corner would have occupied. That falls out of asking for
the glyph with the bits for the sides present, with the plain rule standing in
when only one edge is set — a single edge has no glyph of its own, because that
cell is part of a run, not a corner.

Labels need the rule they sit on

A title and a subtitle live on the top rule, a footer on the bottom. None is
drawn when its rule is absent: painting a title over the first row of content is
worse than leaving it out.

Verified

All six ports, with "all" as the default so every existing frame is
identical — which is what the suites check:

port	result
TypeScript	249 tests, 11 of them new
Rust	green
Go	both packages
Python	36, OK
Zig	17/17, 6/6 steps
C / C++	9/9 ctest

The new tests cover each side alone, adjacent pairs getting exactly one corner,
three sides, none, the empty list, the interior cost per rule, the title rule,
and a check that every border style renders byte-for-byte as before when sides
is "all".

🤖 Generated with Claude Code

https://claude.ai/code/session_017Df2FNu5DhinMV2soRz3cy