Fallback fonts inset or scale U+E0B0 and U+E0B2, so Powerline prompts
showed a gap or a narrow bar where one segment should flow into the next.
Draw those two glyphs as triangles from the cell's own edges, keep them
out of batched text runs, and reuse the same fill when a block cursor
sits on the cell.

Takeover rebuild of the original implementation: one fill helper with
the glyph check at each call site instead of a validating helper that
returns whether it drew; the Powerline-only test is folded into one
renderer integration test.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>