fix: preserve dictionary key comparers across value conversion

[AGiorgetti]

Summary

Fixes #31.

• Preserve a concrete Dictionary source's compatible key comparer when the selected key conversion is language identity, independently of value conversion.
• Use the destination default comparer for custom key conversions, including same-type local/default/external converters. Retain Add collision semantics and existing set behavior.
• Add 31 generator regressions and a shared normally compiled example used by clean-package, framework/language, normal, trimmed, and Native AOT consumers. Synchronize consumer guidance and implementation records.
• No public API or specification semantics change.

Test-first evidence

• On unchanged develop c973aac8e457cc40a4069889aeab49d8278ccca3, the new 31-case C# 9 fixture produced 19 failures and 12 passing controls, zero skips.
• Value widening and structural mapping lost comparer identity; same-type custom key mappings incorrectly retained it. Fixture inputs compile without unrelated errors, and successful generated fixtures are warning-free.
• The normally compiled Collections sample independently built with zero warnings/errors, then failed its comparer-preservation assertion.
• Coverage includes all three dictionary destinations, empty collections, nullable structural values, same-type/widening value converters, local/default/external key converters, independent mutation, Add collisions, interface-declared sources, member/update helpers, null policies, and same-driver deterministic output.

Local validation

• Final Release solution build: zero warnings/errors.
• Full generator: 904 passed; runtime: 5 passed; integration: 1 passed. Zero failures/skips in these projects.
• Focused dictionary/collection/set group: 96 passed, zero failures/skips.
• All three source samples passed. Actual generated source preserves the comparer for identity keys and uses default construction for transformed keys, with one foreach/Add loop.
• Available packaging: 224 passed, two infrastructure failures, zero skips.
• All 12 framework/language combinations compile and execute the shared dictionary example, including genuine netstandard2.0 references, plus net8.0/net9.0/net10.0 and C# 9/14/latest.
• Clean-package samples, usage validation, package/API checks, and benchmark smoke passed.
• Independent read-only review found no scoped blocker. Consumer-skill and CRLF-aware diff checks passed.
• Every uploaded blob and the remote tree matches the tested local source tree 34cc2cfead4977b84e1884c336d66ddf7651afaf.

Local environment limits

Trimming and Native AOT fail at ILLink's ComputeManagedAssemblies task-host creation with MSB4216 / Unix-domain SocketException (13), before publish behavior is established. PowerShell and clang are absent; 49 PowerShell-dependent tests were not run locally. No assertion, warning, dependency, prerequisite, credential, or security setting was weakened.

Validation used the existing official Microsoft SDK 10.0.401 and runtimes 8.0.31/9.0.20/10.0.12, workspace-local caches/temporary files, and one-processor builds.

CI and review

Exact-head CI run 37944436770 succeeded for cc123ef7e1e7dd5078588d5867b0e60a40b90b2e.

• Windows and Linux each passed 1,185 tests: 904 generator, 275 packaging, 5 runtime, and 1 integration; zero failures and zero skipped tests on each OS.
• All six Roslyn jobs (4.8.0, 4.14.0, and 5.9.0 on both OSes) passed 904 tests each, with zero failures/skips.
• Both full jobs cover the locally unavailable PowerShell, trimming, and Native AOT checks. Linux sample/benchmark smoke and both solution-path checks also passed.
• Release packing, artifact verification, and NuGet publication jobs were correctly skipped for this ordinary PR event.

The PR was opened as a draft and subsequently merged by the maintainer at 2026-10-09 14:37:41 UTC as 7b19def74753df90cb3770c7410ff5d7a3951244. No merge, release, tag, NuGet publication, or workflow dispatch was performed by Codex.

Issue #32 and the previously documented nullable-intermediate enum defect remain outside this scope.

Prepared with OpenAI Codex.