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py2js: fix stdlib builtin errors surfacing as "[object Object]" (#330)

Closes #295. bridgeBuiltin (stdlibBridge.ts) calls straight into CSE's own
stdlib implementations (misc.ts, math.ts, ...) at the value boundary. When
one of those raises a runtime error -- handleRuntimeError throws a
RuntimeSourceError -- it propagated uncaught all the way to index.ts's
catch blocks, whose `e instanceof Error ? ... : String(e)` fallback always
hit the String(e) branch: RuntimeSourceError implements SourceError, not
extends Error (see errors/errors.ts), so it's a plain object with no
custom toString(), and String() on that is unconditionally
"[object Object]" regardless of whatever real content .message held.

Fixed at the boundary where the CSE-shaped error actually originates:
bridgeBuiltin now catches a thrown RuntimeSourceError and converts it into
a proper Py2JsRuntimeError, using error.constructor.name (TypeError,
IndexError, ZeroDivisionError, ...) as the kind -- none of these error
classes ever set `.name` themselves, so the constructor is the only
reliable source for it -- and the original .message (CSE's own multi-line
formatted diagnostic) as the message. Not attempting to also clean up that
message's synthetic "at line 1" header here -- stdlibBridge.ts's own file
header already documents py2js not having real error locations through
this bridge yet as a separate, pre-existing gap.


Claude-Session: https://claude.ai/code/session_01KrwJGug3rCXijRseRjys9V

Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>

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