Mark story 4-1 done, add deferred items from review

Review passed all criteria; update sprint-status to done, close the
story file, and record two deferred items (GPIO pins into constants.py,
ButtonHoldDetector init-time GPIO dependency) in deferred-work.md.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

_bmad-output/implementation-artifacts/4-1-gpio-button-hold-detection-and-led-feedback.md

@@ -1,6 +1,6 @@
 # Story 4.1: GPIO Button Hold Detection & LED Feedback
 
-Status: review
+Status: done
 
 ## Story
 

_bmad-output/implementation-artifacts/deferred-work.md

@@ -200,3 +200,17 @@ Description: `time.sleep(REFRESH_INTERVAL_S)` is a dumb fixed sleep appended aft
 Story: `2-7-operational-radar-loop-startup-screen-and-systemd-wiring`
 Category: Technical debt
 Description: The startup screen text is drawn at `DISPLAY_WIDTH // 2 - 60, DISPLAY_HEIGHT // 2` (i.e. x=340). The offset of −60 is a pixel-counted approximation, not derived from actual font metrics. Depending on the Pillow version and the default font's rendered width, the text may appear left-biased. Future hardening: use `ImageDraw.textlength()` (Pillow ≥9.2) to compute the real string width and centre precisely.
+
+---
+
+## Story 4.1: GPIO Button Hold Detection & LED Feedback
+
+### [4-1] GPIO pin numbers belong in constants.py
+Story: `4-1-gpio-button-hold-detection-and-led-feedback`
+Category: Technical debt
+Description: `BUTTON_GPIO_PIN = 17` and `LED_GPIO_PIN = 27` are module-level constants in `gpio_ctrl.py`. For production tuning and hardware revision they belong in `constants.py` alongside other hardware-facing constants. Future hardening: move both to `constants.py` and import them into `gpio_ctrl.py` from there.
+
+### [4-1] ButtonHoldDetector instantiates real gpiozero.Button at __init__
+Story: `4-1-gpio-button-hold-detection-and-led-feedback`
+Category: Infrastructure/environment
+Description: `ButtonHoldDetector.__init__` constructs a `gpiozero.Button` immediately. The radar main loop must construct `ButtonHoldDetector` at startup (not at import time), or the application will fail if GPIO is unavailable when the module is imported. This is currently safe because `gpio_ctrl.py` is not imported at module level in `main.py`, but any future reorganisation that imports it at the top of a module that runs on non-GPIO hardware will raise a `BadPinFactory` error unless a `MockFactory` is active.

_bmad-output/implementation-artifacts/sprint-status.yaml

@@ -70,6 +70,6 @@ development_status:
 
   # Epic 4: Reset & Reconfiguration
   epic-4: in-progress
-  4-1-gpio-button-hold-detection-and-led-feedback: review
+  4-1-gpio-button-hold-detection-and-led-feedback: done
   4-2-config-wipe-setup-screen-and-return-to-provisioning: backlog
   epic-4-retrospective: optional