benchmark cpu: fixed-width duration column

[ThomasWaldmann]

The elapsed time was printed with a variable width ({dt:.3f}s, so 1.823s vs. 92.361s), which misaligned everything right of it — throughput and ratio — as soon as one row took >= 10s. Slow codecs at high compression levels easily do:

$ borg benchmark cpu --compressing --data ~/silesia
Compression ==========================================================
zstd,5 (128kiB)            200.38 MiB  1.823s     115.3 MB/s    3.04x
zstd,10 (128kiB)           200.38 MiB  5.458s      38.5 MB/s    3.16x
zstd,16 (128kiB)           200.38 MiB  36.432s       5.8 MB/s    3.37x
zstd,22 (128kiB)           200.38 MiB  92.361s       2.3 MB/s    3.38x

Now a duration_str() helper reserves 8 chars, right-aligned ({dt:>7.3f}s), which fits up to 999.999s. Applied to both duration columns — report() and the msgpack row, which had the same format — and the section header rule grows 70 -> 71 to keep covering the longest rows again.

After:

Compression ===========================================================
zstd,5 (128kiB)            200.38 MiB    1.823s     115.3 MB/s    3.04x
zstd,10 (128kiB)           200.38 MiB    5.458s      38.5 MB/s    3.16x
zstd,16 (128kiB)           200.38 MiB   36.432s       5.8 MB/s    3.37x
zstd,22 (128kiB)           200.38 MiB   92.361s       2.3 MB/s    3.38x
msgpack ===============================================================
msgpack                    128k Items    0.051s    2522.0 kItems/s

JSON output is unaffected.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.65%. Comparing base (9d831da) to head (e861c72).
⚠️ Report is 16 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10324      +/-   ##
==========================================
- Coverage   87.70%   87.65%   -0.05%     
==========================================
  Files         103      103              
  Lines       18717    18716       -1     
  Branches     2881     2881              
==========================================
- Hits        16415    16405      -10     
- Misses       1600     1609       +9     
  Partials      702      702              

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