add ProgressIndicatorSpinner, for work of unknown duration

[ThomasWaldmann]

The other progress indicators either count (ProgressIndicatorPercent) or just emit a message (ProgressIndicatorMessage). For work whose duration or size is not known in advance, neither fits: what we want to show is "borg is still working, at this" — and that is a spinner.

with ProgressIndicatorSpinner("Deduplicating", msgid="create") as spinner:
    for chunk in chunks:
        process(chunk)
        spinner.show()

How it behaves

• Pull-based on purpose: no thread, no timer, nothing runs while the caller is not calling. show() is rate limited by BORG_PROGRESS_FPS internally, so calling it a million times a second costs one time.monotonic() per call.
• On a terminal: the frame and the message are repainted in place, in borg green (#22D045 on truecolor terminals, else the terminal's own bright green — the nearest xterm-256 entry drops the red channel and reads too acid). The message is truncated to the terminal width, because a wrapped line could not be repainted in place. The cursor is hidden at the first paint and switched back on in finish().
• Without a terminal, or with --log-json: an animation would be pointless, so nothing is repainted — only message changes are logged, as progress_message, exactly like ProgressIndicatorMessage does it. So a logfile gets one line per phase rather than one per frame, and frontends need no changes.
• --quiet silences it, like the other indicators.

Frames

All frames are East Asian Width "Neutral", so they occupy exactly one cell and the message after the spinner never shifts column:

name	frames
square (default)	▫ ▪ ◻ ◼	pulsing: small to medium, hollow to solid
cube	◰ ◳ ◲ ◱	quadrant rotating clockwise
boxed	⊞ ⊠ ⊟ ⊡	squared plus/times/minus/dot
ascii	| / - \	the classic

If the output encoding can not represent the fancy frames, the ASCII ones are used automatically.

New environment variable

BORG_SPINNER=off never animates, BORG_SPINNER=ascii forces the ASCII frames. Documented in the environment help topic (and thus in the docs). Colour follows the usual NO_COLOR / COLORTERM conventions.

Notes

• No caller yet — this adds the indicator only. The existing indefinite-duration spots (e.g. Cache.close()) are single-call phases where nothing would animate; wiring it up where there is a real loop is a separate change.
• Tests cover animation, frame sets and fallbacks, colour, rate limiting, truncation, --quiet, non-tty logging, and a stream that goes away mid-run.

[ThomasWaldmann]

@mr-raj12 use it where appropriate.

I think it can be used to give life-signs at a finer granularity than what we had yet (because it stays in one place, it is fps rate limited). No more "is borg hanging?" questions...

[codecov]

Codecov Report

❌ Patch coverage is 98.05825% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.66%. Comparing base (4f861a3) to head (acf222a).
⚠️ Report is 9 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/borg/helpers/progress.py	98.03%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10081      +/-   ##
==========================================
- Coverage   86.70%   86.66%   -0.04%     
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  Files          98       98              
  Lines       17174    17275     +101     
  Branches     2609     2621      +12     
==========================================
+ Hits        14890    14971      +81     
- Misses       1586     1604      +18     
- Partials      698      700       +2     

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