test: proof infrastructure — snapshot corpus, diff manifest tool, benchmarks, CI

[kamronbatman]

Summary

Groundwork for the upcoming incremental-pipeline and correctness work: before changing the generator, this PR establishes the machinery that proves such changes are output-identical and actually faster. No generator behavior changes (one tool fix aside).

What's included

• Snapshot corpus (Snapshots/, 15 fixtures): pins exact generated output byte-for-byte across the feature surface — fields, UO types, collections, sorted-set comparers, enums, save flags (small/ulong/multi-enum), versioned migrations, inheritance, structs, generics, readonly fields, timer + dirty tracking, field modifiers. Output must also compile against Server stubs aligned with the real interfaces. UPDATE_SNAPSHOTS=1 regenerates; the generator version stamp and line endings are normalized so version bumps and autocrlf don't invalidate the corpus.
• DiffTool: runs the generator over every project in a solution (feeding migration files, unlike the schema pass) and writes a deterministic manifest of hint names + content hashes. Diffing manifests from two generator builds proves output-identical refactors against a real corpus: a 3,786-class solution hashes in ~14s, two consecutive runs are byte-identical, zero generator diagnostics.
• Benchmarks: cold full run vs warm re-run after a single-file edit over a 150-class corpus. Baseline on the current pipeline: the warm re-run costs more than a cold full run (29.2ms / 14.8MB allocated vs 19.3ms / 10.3MB) — every class regenerates and re-parses its migrations on every compilation, plus incremental bookkeeping on top.
• CI: a build-and-test workflow on push/PR — the repo previously had only the manual NuGet publish workflow, so nothing enforced the test suite.
• Fix: SourceCodeAnalysis now accepts .slnx solutions; it refused them outright, which broke the schema tool against solutions using the new format.

Latent defects surfaced by the compile gate (left for the correctness-sweep PR)

• An instance MarkDirty() on the serialized class emits ClassName.MarkDirty()() — fully-qualified display string plus appended parens; never compiles.
• KeyValuePair direct fields emit a != comparison, a .Clear() call, and undeclared key/value locals.
• Struct Serialize/Deserialize are emitted virtual (CS0106) — pinned via the Structs fixture's KnownBroken.txt marker.
• Emitted code references Server-namespace types unqualified (Core, IGenericReader in migration content structs), so it only compiles for classes declared under Server.*; the fixtures mirror that constraint.

Test suite: 79/79 green (64 existing + 15 snapshot fixtures).

🤖 Generated with Claude Code