[derechopiano]

Adds optional, workbook-owned Python projects to the shared macro host: bounded multi-module source, XLSX persistence, Python templates/settings in the existing Developer editor, and cancellable native execution with one undo step and rollback on error/Stop/timeout.

Dependency and scope: this PR targets feat/plugin-host at 3e325c1, the branch behind storytold#1123. It contains only the additions to that host. It implements the desktop project portion of storytold#1566 and leaves that issue open. Form controls, Pyodide, language tooling and rich output remain follow-ups.

The companion provider, sirmmo/craft-python#1, extends craft-python 0.3.0 through its existing App/Channel, preserving Run Script, Console, Run File, PY.EVAL and WordCraft/DeckCraft support. The optional role uses a separate identity so ordinary PY.* formulas remain callable during a macro. It does not implement the Python-in-cells work in storytold#469.

• Adds pythonProjects/engineCommands capabilities while preserving protocol 2.2 and existing VBA/Office.js behavior. Only Python UI runs opt into the new absolute deadline; VBA retains its existing synchronous Run path.
• Keeps Session work on the owner thread; bounded background transport supports absolute deadlines and Stop/Escape. Conflicting edits/saves are blocked and asynchronous execution refuses live collaboration.
• Keeps Python/uv optional. Preparation is offline unless the user approves the exact PEP 723 metadata for this document session; approval is never saved in the workbook. Trusted Python has normal user permissions, not an OS sandbox.

Validation:

• Full local cargo xtask ci: all eight gates passed on Windows, using the real companion provider.
• Native provider tests cover inert discovery, source/workbook isolation, warm reuse, literal writes, ordinary PY.TWICE recalculation, one-step undo and failure/Stop rollback.
• Transport tests cover initialization, deadlines, output flooding, late replies and cancellation; automated UI tests exercise Run/Stop/Escape and settings in light/dark themes, with rendered screenshots inspected. A UI regression verifies existing VBA Run remains synchronous.
• Companion craft-python suite: 68 applicable tests passed on Windows/Python 3.11 and on Linux/Python 3.8 and 3.12, including native process-tree lifetime checks on each platform (three checks for the other platform are inapplicable). Linux CI evidence. FreeBSD CI provisions uv/Python through the existing full workspace test job; native FreeBSD execution remains unverified locally. Linux host CI uses the minimal host-protocol fixture unless the external provider is explicitly selected.

See docs/python-macro-integration.md for installation, APIs and limitations.