feat: support configurable tabline status label

docs/configuration.md

@@ -44,6 +44,20 @@ Plugin paths resolve relative to the config file, not the shell's working direct
 
 `config.diff_context_lines = 10` sets how many unchanged aligned lines appear above and below each diff change; it defaults to `10` and accepts nonnegative integers, including `0` for changes only. `config.diff_expand_lines = 15` sets how many additional lines each directional expansion reveals per keypress; it defaults to `15` and accepts positive integers. Both are bounded by the platform's Rust `usize` maximum (`18446744073709551615` on 64-bit systems). Floats, strings, booleans, negative or oversized values, and a zero expansion step reject the whole Ruby configuration.
 
+### Tabline status label
+
+Set `config.tabline_label` to a plain string for the right side of the top tab line:
+
+```ruby
+Vamp.configure do |config|
+  config.tabline_label = ENV.fetch("VAMP_STATUS", "")
+end
+```
+
+The default `""` disables the label. A nonempty value shows the tab line even with one tab, using the existing accent background and left density fade. The label yields to the active tab when both cannot fit. Unicode is supported; control characters are replaced rather than interpreted as terminal escapes. Non-string values reject configuration.
+
+Configuration runs once at startup. For live values such as battery charge, load a [Ruby plugin](plugins.md#tabline-status-updates) that supplies updates; the editor does not collect battery data or run periodic shell commands itself.
+
 ## Actions and modes
 
 `config.core` exposes the built-in actions below as immutable `Action` values; `config.modes` exposes immutable `Mode` values. Misspelled methods fail during configuration evaluation, and `bind` rejects the wrong value type. Built-in action strings, canonical Ex names (such as `"quit"`), shorthand aliases (such as `"q"`), and mode strings are not accepted. Only plugin-owned commands use exact string names (`"greet"`, without `:` or arguments) and can only be bound in Normal mode. Interactive Ex commands remain textual. Ruby provides runtime checks, not compile-time checking; the editor still validates action/mode compatibility.