Fix framebuffer image memory retention

[mariusandra]

Summary

Fixes framebuffer frames retaining high RSS after rendering large local photos.

• Clear transient render/image source image fields after run and get.
• Drop the final rotated render image reference after driver render.
• On framebuffer devices, force Nim collection and call Linux malloc_trim(0) after each draw so freed decode/scale memory can return to the OS.
• Add render/image tests that assert transient image fields are cleared.

Root Cause

The local image scene decodes full-size photos before scaling them to the framebuffer. On HDMI framebuffer targets with the Pixie engine, large decoded source images and allocator high-water memory could keep process RSS near the systemd memory limit after each render. Later large images then pushed frameos into OOM. E-ink devices were less affected because their render cadence/path differs.

Impact

Framebuffer frames that cycle through large local images should return to a lower memory baseline after each render instead of gradually sitting near MemoryMax. The forced reclamation is scoped to device == "framebuffer" to avoid adding GC work to other driver paths.

Validation

• nim c -r src/apps/render/image/tests/test_app.nim
• nim c -r src/apps/data/localImage/tests/test_app.nim
• nim c -r src/frameos/tests/test_runner_loop.nim
• nim check --os:linux src/frameos/utils/memory.nim
• nim c --compileOnly -d:malloc --nimcache:.nimcache/frameos_check_malloc src/frameos.nim