Conditional breakpoints via VICE checkpoint conditions

## Description

VICE checkpoints support conditions (`has_condition` field in `CHECKPOINT_INFO`). The BMP protocol provides `CMD_CONDITION_SET` (0x16) to attach monitor expressions to checkpoints (e.g., `A == $42`, `.PC == $C000`).

## Proposed implementation

- Add `condition_set(checkpoint_number, expression)` to `ViceBmpClient`
- Add a `set_breakpoint_condition` remote method in `methods.py`
- Display the condition expression as an attribute on breakpoint objects in the trace
- Optionally parse/validate conditions before sending

## Why this matters

Conditional breakpoints are essential for debugging loops and interrupt handlers on the C64, where unconditional breakpoints fire too frequently to be useful.

Look at the ViceBmpClient class and methods.py to understand the existing checkpoint and command structure. The CMD_CONDITION_SET (0x16) command needs to be implemented. Start by adding the condition_set method to ViceBmpClient and the corresponding remote method. Test by setting a condition on a checkpoint and verifying it appears in the trace.

索引モデルが issue の本文から書いたものです。