feat: add configuration system and output window

Introduce config module for managing user options, defaults, and watchexec
binary discovery. Add window module for creating and managing the output
window (float or split) with append, clear, resize, and keymap support.

tests/watchexec/config_spec.lua

@@ -0,0 +1,126 @@
+local config = require("watchexec.config")
+local stub = require("luassert.stub")
+
+describe("watchexec config", function()
+  after_each(function()
+    config.reset()
+  end)
+
+  describe("defaults", function()
+    it("returns default values before setup", function()
+      local cfg = config.get()
+
+      assert.equals("watchexec", cfg.watchexec.bin)
+      assert.same({}, cfg.watchexec.args)
+      assert.equals("float", cfg.window.type)
+      assert.equals("below", cfg.window.split)
+      assert.equals(12, cfg.window.size)
+      assert.is_true(cfg.auto_scroll)
+      assert.equals(5000, cfg.max_lines)
+    end)
+  end)
+
+  describe("setup()", function()
+    it("merges user options over defaults", function()
+      config.setup({
+        auto_scroll = false,
+        watchexec = { args = { "-e", "py" } },
+      })
+
+      local cfg = config.get()
+
+      assert.is_false(cfg.auto_scroll)
+      assert.same({ "-e", "py" }, cfg.watchexec.args)
+      assert.equals("watchexec", cfg.watchexec.bin)
+    end)
+
+    it("returns without changes when opts is nil", function()
+      config.setup(nil)
+
+      local cfg = config.get()
+
+      assert.equals("watchexec", cfg.watchexec.bin)
+    end)
+
+    it("finds watchexec binary via PATH", function()
+      local exec_stub = stub(vim.fn, "executable", function(name)
+        if name == "watchexec" then
+          return 1
+        end
+        return 0
+      end)
+
+      config.setup({})
+
+      local cfg = config.get()
+
+      assert.equals("watchexec", cfg.watchexec.bin)
+
+      exec_stub:revert()
+    end)
+
+    it("falls back to candidate paths when not in PATH", function()
+      local exec_stub = stub(vim.fn, "executable", function(name)
+        return 0
+      end)
+
+      local expand_stub = stub(vim.fn, "expand", function(name)
+        if name == "~" then
+          return "/home/user"
+        end
+        return ""
+      end)
+
+      local has_stub = stub(vim.fn, "has", function(name)
+        return 0
+      end)
+
+      config.setup({})
+
+      local cfg = config.get()
+
+      assert.is_nil(cfg.watchexec.bin)
+
+      exec_stub:revert()
+      expand_stub:revert()
+      has_stub:revert()
+    end)
+
+    it("notifies when binary is not found", function()
+      local exec_stub = stub(vim.fn, "executable", function()
+        return 0
+      end)
+
+      local has_stub = stub(vim.fn, "has", function()
+        return 0
+      end)
+
+      local expand_stub = stub(vim.fn, "expand", function()
+        return "/home/user"
+      end)
+
+      local notify_stub = stub(vim, "notify")
+
+      config.setup({})
+
+      assert
+        .stub(notify_stub)
+        .was_called_with("watchexec.nvim: could not find watchexec binary. Set opts.watchexec.bin in your config.", vim.log.levels.WARN)
+
+      exec_stub:revert()
+      has_stub:revert()
+      expand_stub:revert()
+      notify_stub:revert()
+    end)
+  end)
+
+  describe("get()", function()
+    it("returns the current config table", function()
+      config.setup({ max_lines = 100 })
+
+      local cfg = config.get()
+
+      assert.equals(100, cfg.max_lines)
+    end)
+  end)
+end)