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.

examples/config.rb

@@ -10,7 +10,7 @@ Vamp.configure do |config|
   config.whitespace_characters = {} # opt in: { " " => "·", "\t" => "→", "\n" => "↵" }
   config.whitespace_scope = "all" # or "leading" for indentation only; newline markers remain independent
   config.tab_spaces = nil # opt in: 4 inserts four spaces per Tab
-  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"

src/ruby.rs

@@ -1006,7 +1006,8 @@ mod tests {
             r#"
 Vamp.configure do |c|
   c.tab_spaces = 4
-  c.language_tab_spaces = { "ruby" => 2, "makefile" => nil, "json" => 1, "yaml" => 255, "cpp" => 3, "typescriptreact" => 5 }
+  c.language_tab_spaces.merge!("ruby" => 2, "makefile" => nil, "json" => 1, "yaml" => 255, "typescriptreact" => 5)
+  c.language_tab_spaces["cpp"] = 3
 end
 "#,
         );
@@ -1069,12 +1070,18 @@ end
             for assignment in [
                 format!("c.tab_spaces = {value}"),
                 format!(r#"c.language_tab_spaces = {{ "ruby" => {value} }}"#),
+                format!(r#"c.language_tab_spaces.merge!("ruby" => {value})"#),
+                format!(r#"c.language_tab_spaces["ruby"] = {value}"#),
             ] {
                 let config = fixture.file(
                     "config.rb",
                     &format!("Vamp.configure {{ |c| {assignment} }}"),
                 );
-                assert!(load_config(&config).is_err(), "{assignment}");
+                let error = load_config(&config).unwrap_err();
+                assert!(
+                    error.contains("tab_spaces must be nil or an Integer"),
+                    "{assignment}: {error}"
+                );
             }
         }
         for value in [
@@ -1092,6 +1099,24 @@ end
         }
     }
 
+    #[test]
+    fn language_tab_space_mutations_reject_invalid_keys_before_serialization() {
+        let fixture = Fixture::new();
+        for key in [r#""""#, ":ruby", r#""\n""#, "7"] {
+            for assignment in [
+                format!("c.language_tab_spaces.merge!({key} => 2)"),
+                format!("c.language_tab_spaces[{key}] = 2"),
+            ] {
+                let config = fixture.file(
+                    "config.rb",
+                    &format!("Vamp.configure {{ |c| {assignment} }}"),
+                );
+                let error = load_config(&config).unwrap_err();
+                assert!(error.contains("Tab language"), "{assignment}: {error}");
+            }
+        }
+    }
+
     #[test]
     fn actual_ruby_dsl_context_queued_effects_and_output_isolation() {
         let fixture = Fixture::new();

src/ruby_config.rb

@@ -120,6 +120,9 @@ module Vamp
     # @return [Hash{String => Boolean, Hash{String, Symbol => Object}}] language presets/options
     attr_reader :lsp
 
+    # @return [Hash{String => Integer, nil}] LSP-style language IDs to Tab widths
+    attr_reader :language_tab_spaces
+
     # @return [Vamp::BarConfig]
     attr_reader :bar
 
@@ -265,14 +268,7 @@ module Vamp
     # @param languages [Hash{String => Integer, nil}] LSP-style language IDs to Tab widths
     # @return [Hash{String => Integer, nil}]
     def language_tab_spaces=(languages)
-      raise TypeError, "language_tab_spaces must be a Hash" unless languages.is_a?(Hash)
-
-      @language_tab_spaces = languages.to_h do |language, spaces|
-        language = nonempty_string!(language, "Tab language")
-        raise ArgumentError, "Tab language must not contain control characters" if language.match?(/[[:cntrl:]]/)
-
-        [language, tab_spaces!(spaces)]
-      end
+      @language_tab_spaces = language_tab_spaces!(languages)
     end
 
     # @param lines [Integer] nonnegative context lines around each diff change
@@ -350,7 +346,7 @@ module Vamp
         "highlight_trailing_whitespace" => @highlight_trailing_whitespace,
         "whitespace_characters" => @whitespace_characters,
         "whitespace_scope" => @whitespace_scope,
-        "tab_spaces" => @tab_spaces, "language_tab_spaces" => @language_tab_spaces,
+        "tab_spaces" => @tab_spaces, "language_tab_spaces" => language_tab_spaces!(@language_tab_spaces),
         "top_bar" => @bar.top, "bottom_bar" => @bar.bottom, "labels" => @labels,
         "popup" => { "command" => @popup.command, "search" => @popup.search },
         "diff_context_lines" => @diff_context_lines, "diff_expand_lines" => @diff_expand_lines,
@@ -369,6 +365,19 @@ module Vamp
       spaces
     end
 
+    # @param languages [Hash{String => Integer, nil}]
+    # @return [Hash{String => Integer, nil}]
+    def language_tab_spaces!(languages)
+      raise TypeError, "language_tab_spaces must be a Hash" unless languages.is_a?(Hash)
+
+      languages.to_h do |language, spaces|
+        language = nonempty_string!(language, "Tab language")
+        raise ArgumentError, "Tab language must not contain control characters" if language.match?(/[[:cntrl:]]/)
+
+        [language, tab_spaces!(spaces)]
+      end
+    end
+
     # @param value [Integer]
     # @param label [String]
     # @param minimum [Integer]