The MarkedContentStack implementation was already complete.
All 45 tests pass (20 stack tests + 25 operator parser tests).
Acceptance criteria:
- push_bmc 64 times → all push; 65th emits MARKED_CONTENT_DEPTH_EXCEEDED ✅
- push_bmc N then pop_emc N → empty stack ✅
- pop_emc on empty stack → EmcUnderflow diagnostic ✅
- top_mcid returns Some(mcid) when top has MCID; None when empty ✅
- Unit tests cover push/pop balance, overflow, underflow ✅
- INV-8 (no panic) verified on all stack operations ✅
See notes/pdftract-1qoeb.md for details.
