fix: support mutable language tab configuration

docs/configuration.md

@@ -20,7 +20,7 @@ Vamp.configure do |config|
   config.whitespace_characters = {} # opt in: { " " => "·", "\t" => "→", "\n" => "↵" }
   config.whitespace_scope = "all" # or "leading" for indentation only
   config.tab_spaces = nil # opt in with a fixed number of spaces per Tab, e.g. 4
-  config.language_tab_spaces = {} # overrides: { "ruby" => 2, "makefile" => nil }
+  # config.language_tab_spaces.merge!("ruby" => 2, "makefile" => nil)
   config.file_explorer_side = "right"
   config.file_explorer_view = "tree"
   config.file_picker_view = "tree"
@@ -73,6 +73,17 @@ Markers appear in ordinary and historical buffers and both sides of split/unifie
 
 `config.language_tab_spaces` defaults to `{}`. It overrides the global setting using the same case-sensitive language IDs as `config.lsp`, for example `{ "rust" => 4, "ruby" => 2, "python" => 4, "makefile" => nil }`. Syntax detection and built-in LSP presets share canonical IDs rather than maintaining separate naming conventions; other bundled IDs include `shellscript`, `cpp`, and `typescriptreact` for Bash, C++, and TSX. Display labels such as “Ruby” remain presentation-only. A `nil` override preserves literal tabs for that language, even when the global setting uses spaces. Unlisted or unknown languages and unnamed buffers without a recognized shebang use `tab_spaces`. Detection reuses syntax highlighting's filename, extension, and shebang rules, independently of whether an LSP is enabled.
 
+Like `config.lsp`, `config.language_tab_spaces` exposes a mutable hash: use `merge!` to add or override entries, or `config.language_tab_spaces["ruby"] = 2` to change one entry. Whole-hash assignment with `=` remains supported. Both assignment and the final hash contents are validated; invalid keys or widths inserted through hash mutation reject startup configuration rather than bypassing validation.
+
+```ruby
+config.tab_spaces = 4
+config.language_tab_spaces.merge!(
+  "rust" => 4,
+  "ruby" => 2,
+  "makefile" => nil,
+)
+```
+
 Active snippets retain Tab/Shift+Tab argument navigation; after the final stop, Tab uses the configured insertion setting. Explicit Tab bindings still take precedence, and Ctrl+i remains a Tab alias. Bracketed paste, clipboard paste, completion payloads, and plugin text retain literal tab characters. Insert undo/redo and dot repeat preserve the resolved spaces or tab. Widths other than `nil` or integers in `1..255`, non-hash language overrides, and empty, non-string, or control-containing language keys reject the configuration.
 
 `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.