feat: add configurable whitespace markers

docs/configuration.md

@@ -17,6 +17,7 @@ Vamp.configure do |config|
   config.git_gutter = "bar" # or "symbols", "line_numbers", "none"
   config.scroll_distance = 0 # nonnegative edge distance, or nil to center
   config.highlight_trailing_whitespace = false # true marks line-end whitespace in red
+  config.whitespace_characters = {} # opt in: { " " => "·", "\t" => "→", "\u00a0" => "⍽" }
   config.file_explorer_side = "right"
   config.file_explorer_view = "tree"
   config.file_picker_view = "tree"
@@ -55,6 +56,10 @@ The setting applies to ordinary and historical buffers, split/unified diff panes
 
 `config.highlight_trailing_whitespace` defaults to `false`. Set it to `true` to give trailing whitespace a red background in ordinary and historical buffers and both sides of split/unified diffs, including wrapped and horizontally scrolled text. Spaces, expanded tab cells, and Unicode whitespace at the end of a source line are highlighted; whitespace-only lines are included, but empty lines and padding beyond the text are not. Wrapping does not make interior whitespace trailing. Search, selections, and secondary carets retain their background precedence; diagnostic underlines remain visible. This changes display only, not file contents. Only literal Ruby booleans are accepted; other values reject the configuration.
 
+`config.whitespace_characters` defaults to `{}` (disabled). Assign a hash mapping literal whitespace characters to display markers, for example `{ " " => "·", "\t" => "→", "\u00a0" => "⍽" }` for spaces, tabs, and nonbreaking spaces. Unlisted characters keep their ordinary appearance. Keys and values must be single-character strings; keys must be whitespace with visible terminal width, or tabs, and values must be printable one-cell characters. Line breaks and other control characters, zero-width markers, and wide markers reject the configuration.
+
+Markers appear in ordinary and historical buffers and both sides of split/unified diffs. Tabs and wide Unicode whitespace show the marker in their first source cell and leave remaining cells blank, preserving tab stops, wrapping, horizontal scrolling, and cursor/selection geometry. A clipped first cell hides its marker; empty lines and padding do not gain markers. Whitespace with combining characters remains intact. Syntax colors, search, selections, diagnostics, and trailing-whitespace highlighting still apply. This changes display only: editing, copying, and saving retain the original characters.
+
 `config.git_blame` defaults to `true`; set it to `false` to disable current-line inline Git blame and its background worker. Only literal Ruby booleans are accepted; other values reject the configuration. Toggle it for the running session with `:git_blame.toggle`, its palette action, or a custom binding such as `config.bind "leader b", config.core.git_blame.toggle`. There is no default shortcut.
 
 `config.git_gutter` defaults to `"bar"`: a fat `▌` with one blank column between the line number and marker and another before the text. `"symbols"` uses `+` / `-` / `~` with the same spacing. Added, removed, and modified lines use green, red, and yellow respectively. `"line_numbers"` colors the existing numbers without adding a marker column; the current line retains its cyan/bold caret highlight. `"none"` hides line-gutter repository indicators, not line numbers or picker file statuses. Other modes retain diagnostic number colors independently of repository markers. Unsupported strings, empty strings, and non-string values reject the configuration.