Stack contents display

## Description

The 6502 hardware stack lives at $0100-$01FF. Currently the Stack container in the trace is empty beyond the single frame object. Showing actual stack contents would help users understand what's been pushed (return addresses, saved registers, etc.).

## Proposed implementation

- On each stop, read SP and the bytes from $0100+SP+1 through $01FF
- Display stack entries in the Stack container, annotating likely return addresses (2-byte values that look like code addresses)
- Update on every stop event alongside register refresh

## Limitations

The 6502 has no frame pointer, so true call-stack reconstruction isn't possible. This would be a raw stack dump with best-effort annotation.

## Why this matters

Seeing the stack contents helps debug JSR/RTS sequences, interrupt handlers, and stack corruption issues.