fix: restore x/X/D as modal debug keys, add quit_exits_mode config, track mapped keys exactly

tests/test_config.lua

@@ -4,9 +4,11 @@ local config = require("tiny-debugger.config")
 -- defaults unchanged after empty merge
 local opts = config.merge({})
 assert_eq(opts.keys.continue, "c", "default continue key")
+assert_eq(opts.keys.breakpoint, "x", "default modal breakpoint key")
 assert_eq(opts.global_keys.toggle, "<leader>dd", "default toggle key")
-assert_eq(opts.global_keys.breakpoint, "<leader>dx", "default breakpoint key")
+assert_eq(opts.global_keys.breakpoint, "<leader>dx", "default global breakpoint key")
 assert_eq(opts.help_on_enter, true, "default help_on_enter")
+assert_eq(opts.quit_exits_mode, true, "default quit_exits_mode")
 
 -- partial key override
 opts = config.merge({ keys = { continue = "g" } })
@@ -22,9 +24,9 @@ opts = config.merge({ global_keys = { toggle = "<leader>D" } })
 assert_eq(opts.global_keys.toggle, "<leader>D", "overridden toggle key")
 assert_eq(opts.global_keys.breakpoint, "<leader>dx", "preserved breakpoint key")
 
--- disable a global key
-opts = config.merge({ global_keys = { breakpoint = false } })
-assert_eq(opts.global_keys.breakpoint, false, "disabled breakpoint key")
+-- quit_exits_mode override
+opts = config.merge({ quit_exits_mode = false })
+assert_eq(opts.quit_exits_mode, false, "overridden quit_exits_mode")
 
 -- merge doesn't mutate defaults
 config.merge({ keys = { continue = "z" } })