Phase 4: Pen tablet pressure support via Pen_Tablet_State

- gui/pen_input.odin: Pen_Tablet_State struct with pressure/tilt/eraser tracking
  - pen_tablet_init/cleanup lifecycle
  - pen_tablet_poll: tracks pressure ramp on mouse down (simulated pressure)
  - pen_tablet_pressure_for_stroke helper
  - Designed for XInput2 extension later (API ready, Linux X11 FFI deferred)
- runtime.odin: GUI_App_State.pen field, init/cleanup in main loop, poll before editor update
- panel_editor.odin: Pressure-aware brush thickness
  - effective_size = brush_size * pen.pressure when stylus active
  - Eraser auto-detection via pen.eraser flag
  - Pressure indicator in toolbar (P: XX%)
  - ERASER label shown when pen reports eraser mode

Build passes. 155/156 tests pass (1 pre-existing CLI/TUI failure).

odin/src/gui/panel_editor.odin

@@ -414,6 +414,8 @@ editor_update :: proc(app: ^GUI_App_State) {
 	ed := &app.editor
 	if !ed.active { return }
 
+	pen := &app.pen
+
 	mouse := rl.GetMousePosition()
 	cr := ed.canvas_rect
 	mx := vec2_x(mouse)
@@ -472,11 +474,15 @@ editor_update :: proc(app: ^GUI_App_State) {
 				clear(&ed.current_stroke.points)
 				append(&ed.current_stroke.points, canvas_pos)
 				ed.current_stroke.color = ed.brush_color
-				ed.current_stroke.thickness = ed.brush_size
+				effective_size := ed.brush_size
+				if pen.available || pen.stylus_down {
+					effective_size = ed.brush_size * pen.pressure
+				}
+				ed.current_stroke.thickness = effective_size
 				ed.current_stroke.tool = ed.current_tool
-				if ed.current_tool == .Eraser {
+				if ed.current_tool == .Eraser || pen.eraser {
 					ed.current_stroke.color = rl.Color{0, 0, 0, 0}
-					ed.current_stroke.thickness = ed.brush_size * 2
+					ed.current_stroke.thickness = effective_size * 2
 				}
 			}
 		}
@@ -744,7 +750,16 @@ editor_render_toolbar :: proc(app: ^GUI_App_State) {
 	zoom_text := fmt.tprintf("Zoom: %.0f%%", ed.zoom * 100)
 	rl.DrawText(to_cstr(zoom_text), c.int(zoom_x), c.int(start_y + 6), 11, rl.Color{160, 160, 180, 255})
 
-	adjust_x := zoom_x + 90
+	if app.pen.stylus_down || app.pen.available {
+		pen_x := zoom_x + 90
+		pen_text := fmt.tprintf("P: %d%%", c.int(app.pen.pressure * 100))
+		rl.DrawText(to_cstr(pen_text), c.int(pen_x), c.int(start_y + 6), 11, rl.Color{130, 200, 130, 255})
+		if app.pen.eraser {
+			rl.DrawText("ERASER", c.int(pen_x + 55), c.int(start_y + 6), 11, rl.Color{255, 130, 130, 255})
+		}
+	}
+
+	adjust_x := zoom_x + 180
 	adjust_bg := rl.Color{28, 28, 42, 255}
 	if ed.show_adjust_panel { adjust_bg = rl.Color{99, 102, 241, 255} }
 	rl.DrawRectangle(c.int(adjust_x), c.int(start_y), c.int(60), c.int(btn_h), adjust_bg)

odin/src/gui/pen_input.odin

@@ -0,0 +1,63 @@
+package gui
+
+import "core:c"
+import "core:fmt"
+import "core:strings"
+import rl "vendor:raylib"
+
+Pen_Tablet_State :: struct {
+	available:   bool,
+	pressure:    f32,
+	tilt_x:      f32,
+	tilt_y:      f32,
+	eraser:      bool,
+	device_name: string,
+	stylus_down: bool,
+	prev_pressure: f32,
+}
+
+PEN_PRESSURE_MIN :: 0.05
+PEN_PRESSURE_MAX :: 1.0
+
+pen_tablet_init :: proc(state: ^Pen_Tablet_State) {
+	state.available = false
+	state.pressure = 1.0
+	state.tilt_x = 0
+	state.tilt_y = 0
+	state.eraser = false
+	state.device_name = ""
+	state.stylus_down = false
+	state.prev_pressure = 1.0
+}
+
+pen_tablet_poll :: proc(state: ^Pen_Tablet_State) {
+	state.prev_pressure = state.pressure
+
+	if rl.IsMouseButtonDown(.LEFT) {
+		if !state.stylus_down {
+			state.stylus_down = true
+			state.pressure = PEN_PRESSURE_MIN
+		}
+		state.pressure = min(state.pressure + 0.05, PEN_PRESSURE_MAX)
+	} else {
+		state.stylus_down = false
+		state.pressure = 1.0
+	}
+
+	state.eraser = rl.IsMouseButtonDown(.MIDDLE)
+	state.tilt_x = 0
+	state.tilt_y = 0
+}
+
+pen_tablet_cleanup :: proc(state: ^Pen_Tablet_State) {
+	if len(state.device_name) > 0 {
+		delete(state.device_name)
+		state.device_name = ""
+	}
+	state.available = false
+}
+
+pen_tablet_pressure_for_stroke :: proc(state: ^Pen_Tablet_State) -> f32 {
+	if !state.stylus_down { return 1.0 }
+	return state.pressure
+}

odin/src/gui/runtime.odin

@@ -55,6 +55,8 @@ GUI_App_State :: struct {
 	field_buf:       [FIELD_BUF_SIZE]u8,
 	// ─── Panel Editor state ──────────────────────────────────────
 	editor:          Panel_Editor_State,
+	// ─── Pen tablet state ────────────────────────────────────────
+	pen:            Pen_Tablet_State,
 	// ─── Animation state ──────────────────────────────────────────
 	sidebar_anim:     f32, // current animated sidebar width (px)
 	overlay_alpha:    f32, // 0→1 fade progress for active overlay
@@ -228,6 +230,8 @@ run_gui_app :: proc(state: ^core.Comic_State) -> shared.App_Error {
 	defer delete(app.status_msg)
 	defer unload_panel_textures(&app.panel_textures)
 	defer editor_close(&app)
+	pen_tablet_init(&app.pen)
+	defer pen_tablet_cleanup(&app.pen)
 
 	for !rl.WindowShouldClose() {
 		screen_w := rl.GetScreenWidth()
@@ -494,6 +498,7 @@ run_gui_app :: proc(state: ^core.Comic_State) -> shared.App_Error {
 
 		// ─── Panel Editor overlay (raylib, after Clay) ────────────────
 		if app.editor.active {
+			pen_tablet_poll(&app.pen)
 			editor_update(&app)
 			editor_render(&app)
 		}