feat(config): allow view glyph customization

[barrettruth]

Summary

• Add view.change_bar and view.rail_separator config fields.
• Use change_bar for generated unified change bars and split endpoint sign bars.
• Use rail_separator for generated line-number rails, including separator-width metadata so ASCII separators parse and highlight correctly.
• Document the new fields in vimdoc.

Validation

• nix develop .#ci --command stylua --check lua spec
• nix develop .#ci --command vimdoc-language-server check doc/
• nix develop .#ci --command busted spec/config_spec.lua spec/rails_spec.lua spec/highlight_spec.lua spec/commands_spec.lua spec/parser_spec.lua spec/merge_spec.lua spec/read_buffer_spec.lua
• nix develop .#ci --command just lint
• git diff --check