Carry rounded lengths into kilometres

[machines-6657]

Fix format_length() rounding into the next unit

Problem

format_length() chooses a unit before rounding its value to two decimal places. As a result, a supported length such as 999.999 metres is rendered as 1000 metres, even though the rounded quantity is exactly 1 km and that unit is supported.

Change

After rounding, carry into the next larger supported length unit when the rounded value reaches its divider. Add a regression assertion for the metre-to-kilometre boundary.

Validation

• Regression test: fails before the fix (1000 metres instead of 1 km), passes after it.
• Portable suite: 63 passed; six unrelated platform-dependent tests deselected.
• Full suite: 63 passed, 6 failed on Windows due to missing HOME, hard-coded bash, and Unix-only termios dependencies.
• flake8, Python compilation, and git diff --check pass.