fix: correctness sweep for latent emission and pipeline defects

[kamronbatman]

Summary

Fixes every latent defect surfaced by #47's compile gate, plus the pipeline hygiene issues found during the incrementality analysis. Each fix is pinned by a new snapshot fixture or diagnostic test that failed before the fix (6 red tests → all green).

Fixes

• MarkDirty()() emission: an instance MarkDirty() on the serialized class emitted ClassName.MarkDirty()() (ToDisplayString() already includes qualification and parens). Now emits the method name with the proper receiver. Pinned by the CustomMarkDirty fixture.
• KeyValuePair fields (three defects, KeyValuePairField fixture):
  • setters compared with a nonexistent != — setters now fall back to EqualityComparer<T>.Default.Equals for types without an inequality operator (with Nullable<T> operator lifting respected, so int? fields are unchanged);
  • deserialization assigned to undeclared key/value locals — now declared with unique names so multiple pair fields can't collide;
  • a bogus ClearX() calling KVP.Clear() was emitted — data-structure methods now require an actual array/collection/dictionary type.
• Struct emission: Serialize/Deserialize were marked virtual (CS0106). Fixing that exposed that the SG3009 contract was unsatisfiable: it required a user-declared Deserialize, which always collides with the generated one (both flavors, CS0111), while the migration rules consume the generated instance method. SG3009 is inverted: it now fires when a value type declares a conflicting Deserialize. The Structs fixture's KnownBroken marker is gone.
• Unqualified Server-namespace references: migration content structs (IGenericReader) and timer emission (Core.Now) only compiled for classes under Server.*. Both fully qualified now — pinned by the new NonServerNamespace fixture, which generates a versioned class with a timer outside Server.* and must compile.
• Thread safety: the generator's Migrations map is mutated from source-output callbacks that can run concurrently — now a ConcurrentDictionary. Also removed the no-op CultureInfo set/restore in Initialize.
• New diagnostics: SG3011 (duplicate migration file for the same class+version was silently ignored — data-loss hazard with nested migration folders) and SG3012 (migration file above the current version is never read — leftover from a rolled-back bump). The file at the current version is the schema record the migration tool maintains and is not flagged; warnings now survive successful generation instead of being dropped.

Proof

• Suite green: 83/83 (6 new red tests flipped by the fixes).
• DiffTool manifest vs. main across a 3,786-source real corpus: exactly 25 hashes change — all migration content structs and timer classes picking up qualified names — and zero diagnostics fire, confirming no real-world duplicate or stale migration files and no unintended output drift. (An earlier draft of SG3012 used >= and lit up the entire corpus — the manifest run caught it immediately.)

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