Fix #14: Playground is too persistent, errors are not explained

[leftibot]

Automated fix by @leftibot.

What changed

Fix #14: Reset playground engine between runs and display errors
The ChaiScript WASM engine accumulated state (variables, functions) across
script executions, causing confusing errors when editing code. Additionally,
if the engine aborted due to an unhandled exception, no meaningful error was
shown and subsequent runs silently failed.
Each run now reloads the WASM module via resetEngine() to guarantee a clean
engine state. An onAbort handler catches WASM crashes and displays the error
message. Exception formatting handles string, Error, and unknown throw types.
Co-Authored-By: Claude Opus 4.6 (1M context) noreply@anthropic.com

Files

 playground.html    | 96 ++++++++++++++++++++++++++++++++++++++++++------------
 test_playground.sh | 10 +++++-
 2 files changed, 85 insertions(+), 21 deletions(-)

Closes #14

Triggered by @lefticus.