Feature/2621 add export name for joined exports

[sheddy123]

#2621
These implementations were carried out:
Added a configurable Title on the config, used as the joined summary's title. This drives the exported file names (via ExporterBase.GetFileName → summary.
Title for multi-type/joined summaries). When unset, the existing BenchmarkRun-joined-{timestamp} default is used, so behavior is unchanged for everyone else..

You can now set it two ways:

// Fluent API
ManualConfig.Create(DefaultConfig.Instance).WithTitle("MyBenchmarks");
// CLI
--join --title=MyBenchmarks

Scenario	How title is set	Exported files	HTML <title>
Code API	.WithTitle("MyCustomBenchmarks")	MyCustomBenchmarks-report.csv / -report-github.md / -report.html	MyCustomBenchmarks
CLI	--join --title CliCustomTitle	CliCustomTitle-report.csv / ...	CliCustomTitle
Default (unchanged)	(nothing)	BenchmarkRun-joined-2026-07-29-15-25-19-report.csv / ...	BenchmarkRun-joined-2026-07-29-15-25-19

[sheddy123]

@timcassell this is up for approval/merge

[timcassell]

Does it not affect the title of non-joined summaries? Should it?

[sheddy123]

@timcassell
It doesn't config.Title is only read for Summary.Join; non-joined summaries still get their title from GetTitle(...).

[timcassell]

I think it should. The simplest fix looks like would be to prepend the title in the single type case in

BenchmarkDotNet/src/BenchmarkDotNet/Exporters/ExporterBase.cs

Lines 57 to 58 in 95d469d

	if (targets.Length == 1)
	return FolderNameHelper.ToFolderName(targets.Single());

$"{summary.Title}-{FolderNameHelper.ToFolderName(targets.Single())}"

With an added benefit of preventing overwriting multiple run results (the same way the joined title behaves).

Also, this option should override the log file name. Currently it looks like the log file uses different logic and results in a different name than the exported files, it might be good to unify them.

[sheddy123]

@timcassell
Based on the literal suggestion, for a non-joined run, summary.Title is already {FullTypeName}-{timestamp}
so $"{summary.Title}-{ToFolderName(targets.Single())}" produces:

BenchmarkDotNet.Samples.IntroBasic-20260730-101112-BenchmarkDotNet.Samples.IntroBasic-report.csv

The type name lands twice whenever no custom title is set. The prepend only adds information when the title is custom.

[timcassell]

We should redesign how it works, then. The implementation for ExporterBase.GetFileName looks pretty hacky as it is. It's fine if it requires breaking changes to get it right.

[sheddy123]

@timcassell

This was the redesign I did

The design

Summary.Title is now the single source of truth; the runner guarantees it is unique within a run, and GetArtifactFullName is one expression over it; the heuristic is gone. All the rules live in the new Helpers/TitleHelper.cs:

• custom title set - used verbatim; -{TypeName} appended only when the run reports more than one summary, so their files can't collide
• no custom title - {FullTypeName} for a single type, BenchmarkRun-joined for a joined summary
• no timestamp in the base name; ToFolderName already includes the namespace so No namespace in export filenames can lead to data loss #529 is covered, and DontOverwriteResults (timestamped folder) already owns run retention
• the .log file stays {title}-{timestamp}.log, since --resume has to find previous runs; GetIdToResume now matches {title}-*.log
• titles are escaped for path validity and truncated against a budget that accounts for the results folder, so export paths are protected too

I have included a screenshot of testing this locally. Let me know if this implementation suffices