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fix: buffer stdout so multiple print statements all appear (#176)

* fix: buffer stdout and flush as a single sendOutput to fix multiple prints

When print() was called multiple times, only the first line appeared.
Root cause: each sendOutput call sends a separate postMessage to the
host, where the frontend's redux-saga handleStdout task processes one
message per iteration. The STATUS STOPPED message arriving after all
output would trigger task cancellation, but the saga scheduler could
cancel handleStdout between iterations, dropping remaining messages.

Fix: collect all stdout writes into a local buffer during evaluation and
flush the entire buffer as a single sendOutput call after collectSnapshots
completes. One message = one handleStdout iteration = no race condition.
The flush also runs in the catch path so partial output before an error
is still shown.

* fix: clear buffer after flush and add tests for createBufferedOutputStream

- Reset buffer to "" after sendOutput so double-flush (try path + catch
  path when sendSnapshots throws) never duplicates output
- Use strict !== "" check instead of truthy to match the guard condition
  unambiguously
- Add streams.test.ts with 3 unit tests covering: single-flush batching,
  no-op on empty buffer, and idempotent double-flush

Addresses Gemini review comment on PR 176 and restores coveralls coverage.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

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