Problem: vim.json.encode fails with "excessively sparse array" when
extmark row numbers are used as integer table keys, since they create
gaps in the array.
Solution: use tostring(row) as keys instead. Also add hl_eol to dumped
extmark fields for debugging line background extmarks.
line_hl_group bg occupies a separate rendering channel from hl_group in
Neovim's extmark system, causing character-level bg-only highlights to be
invisible regardless of priority. Switching to hl_group + hl_eol ensures
all backgrounds compete in the same channel.
Also reorders priorities (Normal 198 < line bg 199 < syntax 200 < char
bg 201), bumps char-level blend alpha from 0.4 to 0.7 for visibility,
and adds debug logging throughout the intra pipeline.
