[feat] Support multiple aggregations and statistics at once when querying performance data

[vkarak]

This PR offers the following features:

1. Multiple statistics can be presented at once using the special stats aggregator. These includes mean, standard deviation, and 0.01, 0.05, 0.50, 0.95, 0.99 quantiles.
2. The cmp_spec syntax is updated to support multiple statistics at once, e.g., mean,min,max, and also aggregations on values other than the performance, e.g., min(num_tasks),min(pref) etc.
3. Two new command-line options are added to control the name of the left and right terms in the comparisons. These are the --term-lhs and --term-rhs and are only meaningful with the --performance-compare and --performance-report options.
4. The default name suffixes for left/right terms in comparisons are now (lhs) and (rhs) respectively.
5. The selectors of left and right columns in the cmp spec are now _L and _R instead of _A and _B.

Closes #3436.

Implementation details

The analytics layers is now using Polars through its Python bindings.

Managing the column names was the most challenging part of the implementation, as we wanted to keep a straightforward for selecting test attributes from the command line, specifying the left/right variants quickly and the same time produce tables with meaningful headers (e.g., include the aggregation in the column name, especially now, that multiple aggregations are supported at once). This is handled the the internal _QueryMatch class which replaces the very thin former _Match named tuple.

Todos

• Update documentation
• Update tutorial examples
• --table-format=pretty still uses | as a separator for join_uniq.

Notes

If the CI does not pass for Python < 3.9, we should simply merge this PR after #3583 is addressed.

This PR depends on #3596.

[codecov]

Codecov Report

❌ Patch coverage is 95.39749% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.40%. Comparing base (8858b80) to head (cc60ca8).
⚠️ Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
reframe/frontend/reporting/utility.py	94.54%	9 Missing ⚠️
reframe/frontend/reporting/__init__.py	96.72%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3585      +/-   ##
===========================================
+ Coverage    91.38%   91.40%   +0.01%     
===========================================
  Files           62       62              
  Lines        13484    13505      +21     
===========================================
+ Hits         12323    12344      +21     
  Misses        1161     1161              

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[ekouts]

Haven't finished my testing, but stats seems to be broken for --performance-report:

./bin/reframe -c my_checks/perf_vars.py -r --performance-report "now-1d:now/stats:/"
ReFrame Setup]
  version:           4.10.0-dev.0+cc60ca88
  ....
[==========] Finished on Mon Jan 26 11:38:45 2026+0100
WARNING: failed to generate performance report: "pval (std)" not found
Current session stored with UUID: e0a328f6-0ff3-4eeb-85fb-eb0047f17040
Log file(s) saved in '/var/folders/19/lkdv1y8949g05ynf917f07mm0000gn/T/rfm-a76sgaqq.log'

When the database is disabled, reframe doesn't complain about this, just returns the default results.