feat: support units for progress bars

[bepri]

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Adds support for units in progress bars.

Since more sophisticated progress bars are coming from #426, this is an intentionally simple implementation to accommodate other work until the other PR can be released.

CRAFT-5029

[cmatsuoka]

Looks good

[Copilot]

Pull request overview

Adds basic “units” support to progress bars so callers can label the displayed progress/total values (e.g., 2/3 syllables), as a lightweight bridge until the more comprehensive progress bar work in #426 lands.

Changes:

• Add units support to Emitter.progress_bar(...) and thread it through _Progresser → Printer.progress_bar(...).
• Render the units label in terminal progress bar output (... 2/3 <units>).
• Update/extend unit tests, examples, and changelog to cover/document the new parameter.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
craft_cli/printer.py	Stores units on bar messages and appends them to the numeric progress display in terminal output.
craft_cli/messages.py	Adds units to the Emitter.progress_bar API and propagates it through _Progresser.
tests/unit/test_printer.py	Adds a unit test asserting units are displayed in terminal progress bars; updates existing progress_bar calls for new signature.
tests/unit/test_messages_helpers.py	Updates _Progresser tests to pass through and assert units=None.
examples.py	Demonstrates progress bars with units in an example.
docs/changelog.rst	Documents the new units parameter under the next release entry.

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[Copilot]

Printer.progress_bar() now requires the units keyword argument. Even though Printer isn’t exported from craft_cli.__init__, it is still a concrete class in a public module and this is an avoidable breaking change for any direct users/tests. Consider giving units a default of None (and keeping it keyword-only) so existing call sites continue to work without modification.

Suggested change

	units: str | None,
	units: str | None = None,

[bepri]

I don't mind the change since it is public in a strict sense. But I'm pretty sure it's entirely private in practice.

[medubelko]

Looks neat!

Probably an out-of-scope question, but when the unit string is very long, all progress is clipped:

boop
boop
Crunching the numbers

Could we still show the numbers when we clip the units?

boop
boop
Crunching the numbers [59/60]