docs: add memory debugging guide

[cosven]

Summary

• Add a developer guide for memory debugging with Memray and native/system tools.
• Document the minimal Qt baseline step before attributing resident memory to FeelUOwn.
• Record macOS vmmap interpretation notes for Physical footprint, Writable resident, TOTAL resident, and USS-style estimates.

Test Plan

• git diff --check -- docs/source/index.rst docs/source/memory_debugging.rst
• uv run sphinx-build -b html docs/source docs/_build/html

Notes: Sphinx build succeeds with existing warnings, including missing PyQt6 in this isolated worktree virtualenv for GUI autodoc imports and existing docstring/reference warnings.