Add MEMORY-PRINCIPLES.md — definitive memory design reference for native stack

Comprehensive document covering:
- Philosophy: parse-once/share-everywhere, lazy parsing, streaming, Cow errors
- Core primitives: Arc<[u8]>, ArcSlice, ArcStr, Arc<str>, LazyHeaderMap, CoseData
- Operation memory profiles with O() analysis for parse/sign/verify
- Cross-layer patterns: Rust → FFI → C → C++ data flow and ownership rules
- Structurally required allocations inventory with justification
- Allocation review checklist for PR reviews

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Jstatia