feat(SourceGenerator): Scaffold + [<ArguGenerate>] marker attribute

[dimension-zero]

feat(SourceGenerator): Scaffold + [] marker attribute

• New project src/Argu.SourceGenerator (netstandard2.0): publishes the
  [] marker attribute. Today the marker is inert; a
  follow-up release will ship the actual generator that consumes it to
  emit a compile-time-built schema (bypassing Argu's reflection path).
  README.md documents the planned design and roadmap.
• New sample samples/Argu.Samples.SourceGenerated: shows the opt-in
  shape. The sample currently runs through Argu's standard reflection
  path; once the generator lands, the same SampleArgs annotated with
  [] will use the generated factory automatically.
• Argu.sln: register both new projects. The sample is nested under the
  existing 'samples' folder; the SourceGenerator library is a sibling
  of Argu.

Strictly additive. No change to the core Argu package's public surface,
behaviour, or dependency graph.

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[Copilot]

Pull request overview

This PR scaffolds a new companion package (Argu.SourceGenerator) that introduces an [<ArguGenerate>] marker attribute to create an opt-in seam for a future compile-time schema generator, and adds a sample that demonstrates the intended usage pattern.

Changes:

• Adds new src/Argu.SourceGenerator project (netstandard2.0) publishing the [<ArguGenerate>] marker attribute plus package README.
• Adds new samples/Argu.Samples.SourceGenerated sample showing annotation and current reflection-based parsing flow.
• Registers both projects in Argu.sln.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/Argu.SourceGenerator/README.md	Documents motivation, status, and roadmap for the planned generator.
src/Argu.SourceGenerator/AttributeMarkers.fs	Introduces the [<ArguGenerate>] marker attribute type.
src/Argu.SourceGenerator/Argu.SourceGenerator.fsproj	Adds the new packable library project and packaging configuration.
samples/Argu.Samples.SourceGenerated/Program.fs	Adds a sample CLI template annotated with [<ArguGenerate>].
samples/Argu.Samples.SourceGenerated/Argu.Samples.SourceGenerated.fsproj	Adds the new sample project referencing Argu + the marker package.
Argu.sln	Includes the new library and sample in the solution.

---

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[bartelink]

Approved in principle - ideally any forward looking statements like 'planned' get trimmed (see copilot comments) and we merge this just before the impl arrives.

(Main concern being not to merge something that nobody on the planet will end up using if anything changes)

[dimension-zero]

@bartelink @nojaf — packaging hygiene cleanup pushed (ad7c6fc) to address Copilot's review:

• AttributeMarkers.fs: removed the "(shipped separately)" wording that contradicted the <remarks> saying the generator is intentionally out of scope for this release.
• Argu.SourceGenerator.fsproj:
  • Added <PackageReadmeFile>README.md</PackageReadmeFile> so NuGet clients render the README.
  • Added DotNet.ReproducibleBuilds PrivateAssets="All" to match the core Argu package and keep nupkg metadata consistent across the two packages.
  • Changed PackagePath="\" → "" on the README pack item to match the existing pattern in src/Argu/Argu.fsproj.

Pack succeeds with no warnings. CI green. Ready for review whenever convenient — this PR is just the marker scaffold; the generator itself is deliberately out of scope and there's no behaviour change to the core library.

[dimension-zero]

@bartelink — doc comment reworked per your note (cc8b9b5):

• <summary> is now in final form: states the purpose tersely (opts a template into compile-time schema generation → reflection-free ArgumentParser<'T>, which is what makes publish-AOT viable and removes the reflection startup cost), and calls out the key consideration a user must account for — the union and its Argu attributes have to be statically resolvable, since the generator reads the type at compile time. No "planned/future" wording.
• <remarks> carries the caveat that the generator doesn't exist yet and that the marker can be applied now for a no-source-change benefit once it lands.
• README intro reworded into plain English (now vs later).

I've kept this PR marker-only as before. On your "do the generator in one shot" offer — happy to, once you accept the marker first and bring the generator as its own PR so the scaffold isn't held up behind a much larger change. If you'd prefer them together, say so and I'll fold it in. Otherwise this is ready for merge whenever you're happy.

[bartelink]

(merging shortly)

[bartelink]

Thanks for the cleanups & follow-up!