Add net10 Solution.g codegen console + toggle (groundwork to free the SourceGenerators ns2.0 cone)

[ChrisonSimtian]

Groundwork for #441 (move the Roslyn generator off the netstandard2.0 floor). Adds a net10 pre-build codegen path for the strongly-typed [Solution] accessor so the in-compiler generator can eventually shed its Solution/Persistence/Utilities/Globbing dependency cone. Non-breaking and dogfooded.

Warning

Behavior note — opt-in path only, not an API break. Under FalloutSolutionCodegenMode=Build, Solution.g.cs is produced by a pre-build step instead of the in-compiler generator, so live in-IDE regeneration on Visual Studio / Windows (design-time build + IntelliSense) can differ — the accessor refreshes on build rather than on edit. The default path (in-compiler generator) is unchanged, so this affects only builds that opt in (today: this repo's _build). Verifying VS design-time behavior on Windows before any consumer default is flipped.

What changed

• Fallout.Solution.Codegen (net10 console) — discovers [Solution(GenerateProjects = true)] syntactically (no compilation pre-build), reads the solution with the real Fallout.Persistence.Solution parser, emits <Member>.g.cs.
• Fallout.Solution.Codegen.Emit (new netstandard2.0;net10.0 project) — the shared emitter, referenced by both the console and the generator. Replaces the old src/shared/ linked-source hack; output stays byte-identical. Bundled into the analyzer via the existing GetDependencyTargetPaths.
• StronglyTypedSolutionGenerator — self-suppresses when FalloutSolutionCodegenMode == Build; otherwise unchanged, so it remains the in-IDE fallback.
• Fallout.Solution.Codegen.targets — reusable pre-build target + toggle plumbing (FalloutSolutionCodegenProject in-repo, or FalloutSolutionCodegenAssembly for packaged consumers).
• build/_build.csproj — dogfoods the console path (Mode=Build).
• Tests — Fallout.Solution.Codegen.Specs covers discovery (qualified/aliased names, relativePath:, properties/fields, multiple members, negatives), the emit pipeline, stale-output removal, the parameters-file fallback, and identifier escaping; plus generator self-suppression specs and generator/console output-parity specs. Discovery also prunes bin/obj/hidden in the bare---root fallback scan.

Why

The generator parses solutions at compile time, which forces the parser cone to netstandard2.0. Moving codegen to a net10 console lets the generator later drop those refs. Shipped as non-breaking groundwork: both paths coexist (toggle-gated), so consumers are untouched until the console ships in the NuGet.

Verification

• Full fallout.slnx build: 0 errors.
• Fallout.Solution.Codegen.Specs incl. new GeneratorConsoleParitySpecs (drives the in-compiler generator and the console over one fixture; asserts identical + correct Solution.g.cs across attribute spellings, fancy names, and the GenerateProjects=false no-op); Fallout.SourceGenerators.Specs incl. the generator Verify snapshot.
• Dogfood _build: console emits Solution.g.cs → generator suppresses → compiles.

Part of #441.

Follow-ups (not here)

• Ship the console + targets (and the .Emit DLL) in the consumer NuGet (build/), flip the consumer default to Build, then drop the generator's cone refs (frees Solution/Utilities to net10) and the .Emit ns2.0 target.
• The .targets happy path is covered by the dogfood _build (+ Fallout.Canary), not an automated spawn-build test.
• The pre-build target passes every compile item as --source on one Exec line; a very large Mode=Build consumer could hit the Windows ~32K command-line limit (the --root fallback mitigates). Batch/response-file if that ever bites.
• FancyNames is currently dead code — tracked in [Solution] FancyNames option is dead code #445.

[Copilot]

Pull request overview

Groundwork for issue #441 by introducing a net10 pre-build codegen path for the strongly-typed [Solution] accessor (Solution.g.cs), sharing a single emitter between the console path and the Roslyn generator fallback, and dogfooding the build-path in build/_build.csproj.

Changes:

• Add Fallout.Solution.Codegen (net10 console) + MSBuild .targets to generate *.g.cs before compile when FalloutSolutionCodegenMode=Build.
• Extract shared Scriban emit logic to src/shared/SolutionEmitter.cs and link it into both the console and StronglyTypedSolutionGenerator.
• Update solution + Verify snapshot and wire the dogfood build to use the new build-time codegen path.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
tests/Fallout.SourceGenerators.Specs/StronglyTypedSolutionGeneratorSpecs.Test#Solution.g.verified.cs	Snapshot update reflecting the new Fallout.Solution.Codegen project presence.
src/shared/SolutionEmitter.cs	New shared emitter used by both generator and console to keep output byte-identical.
src/Fallout.SourceGenerators/StronglyTypedSolutionGenerator.cs	Refactor onto shared emitter + self-suppress when FalloutSolutionCodegenMode=Build.
src/Fallout.SourceGenerators/Fallout.SourceGenerators.csproj	Link in shared emitter source file.
src/Fallout.Solution.Codegen/Program.cs	New net10 console that syntactically discovers [Solution(GenerateProjects=true)] and emits *.g.cs.
src/Fallout.Solution.Codegen/Fallout.Solution.Codegen.csproj	New net10 console project + linked emitter + references/packages.
src/Fallout.Solution.Codegen/build/Fallout.Solution.Codegen.targets	New MSBuild target to run the console pre-build and include generated sources.
fallout.slnx	Adds the new Fallout.Solution.Codegen project to the repo solution.
build/_build.csproj	Dogfoods the console path (Mode=Build) and imports the new targets.
AssemblyInfo.cs	Adds InternalsVisibleTo("Fallout.Solution.Codegen").

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[ChrisonSimtian]

@dennisdoomen this one would be good, groundwork PR for other stuff