CLI startup optimization: avoid COM availability probing during parser construction

[harumiWeb]

Before submitting

• I understand that this project is maintained in spare time and that not all issues may result in changes.

Description

Summary

The CLI currently performs a COM availability check during startup, and on Windows this may instantiate xlwings.App() and effectively launch Excel.
Because this can happen even for lightweight commands such as exstruct --help, it significantly degrades perceived CLI responsiveness.

To improve startup UX, we should stop probing COM availability during parser construction and only evaluate it when a COM-dependent feature is actually used.

Problem

At the moment, COM availability is checked while building the argument parser. This creates several issues:

• Heavy initialization can run even for --help or argument validation failures
• On Windows, startup may involve launching and quitting Excel
• Users experience the CLI as “slow before doing anything”
• Startup latency becomes more variable and less predictable

In particular, options such as --auto-page-breaks-dir do not need to be conditionally exposed based on a startup-time environment probe.
This is better handled as runtime validation.

Proposal

• Remove COM availability probing from build_parser()
• Always expose --auto-page-breaks-dir as a CLI option
• Validate COM availability only when that option is actually used and a COM-dependent execution path is reached
• If COM is unavailable, fail at execution time with a clear and explicit error message

Expected benefits

• Significantly faster startup for commands like exstruct --help
• Cleaner separation between CLI bootstrap and execution-time capability checks
• Better first-response UX
• Fewer side effects during startup

Scope

Potential files:

• src/exstruct/cli/main.py
• src/exstruct/cli/availability.py

Acceptance criteria

• exstruct --help does not trigger COM availability probing
• Parser construction does not launch Excel
• --auto-page-breaks-dir is validated at execution time instead
• In COM-unavailable environments, the CLI returns a clear and actionable error message

Notes

README wording may also need to be updated from:

• “the option is shown only when supported”

to:

• “the option is always shown, but validated at runtime based on environment support”

Minimal example (optional)

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Additional notes (optional)

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