F-32: Main screen LVGL tree leaks on every return to it #35

Severity: MEDIUM · Effort: Small · Batch: 1 · Status: Code done in #21 (7c980f9), awaiting bench verification

Found in the second full review of the firmware. Recorded with the rest of the pass (F-19…F-44) in docs/REVIEW_REMEDIATION_PLAN_2.md, which is where its status is kept.

Description

lv_scr_load does not free the previous screen — the comment in MainScreen::~MainScreen
says it does — and nothing else freed the main screen's tree. Every return to it leaked four
meters, the power bar, the carousel and a 29-button function panel. Small LVGL allocations come
from internal RAM first, the same heap task stacks and WiFi need. F-03's "20 round trips"
check was never ticked.

Plan

Closed by F-21's fix: the main screen is built once and re-shown.

Acceptance Criteria

• Bench: heap_caps_get_free_size(MALLOC_CAP_INTERNAL) is stable across twenty
  Settings → Back round trips.