fix: auto-bootstrap Python engine before starting bridge (#25)

* fix: auto-bootstrap Python engine before starting bridge

Bridge.start() now calls ensureEngine() to download uv, create an
isolated venv, and install altimate-engine before spawning the Python
subprocess. resolvePython() also checks the managed venv path so
the correct interpreter is used after bootstrapping.

Previously, resolvePython() would fall through to system python3
which doesn't have altimate_engine installed, causing
ModuleNotFoundError on first run.

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

* test: add bridge client tests for ensureEngine and resolvePython

- Export resolvePython() from client.ts for direct unit testing
- Test that ALTIMATE_CLI_PYTHON env var takes highest priority
- Test that managed engine venv is used when present on disk
- Test fallback to python3 when no venvs exist
- Test that ensureEngine() is called before bridge spawn
- Mock only bridge/engine module to avoid leaking into other tests

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

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

Author: anandgupta42