Fix nullable intermediate enum source paths

[AGiorgetti]

Summary

• Preserve effective source-path nullability when lowering built-in enum mappings, separately from the leaf enum symbol's type.
• Reuse the nullable enum wrapper for required-member initialization when a patch update constructs a missing destination, without reselecting converters.
• Add runtime regressions, a shared source/package consumer example, and update the documented limitation. Public API and specification semantics are unchanged.

Fixes #48.

Test-first evidence

• On unchanged production: 28 failures and 42 passing controls in the final 70-case enum fixture; the normally compiled Basic sample also reproduced the missing-path failure.

• Four additional required nullable enum initializer cases failed the first candidate, while the explicit nullable-input converter control passed.

• Final focused validation: 184 passed, zero failures or skips. Cases cover create/constructor/update/patch, by-name/by-value, known/unknown/null values, nullable leaves, reference paths followed by value/tuple fields, changing getters, stable exception metadata, numeric policy, converter precedence, and sequential update effects.

• Initial CI exposed five Windows-only fixture failures caused by an LF-only insertion anchor. The follow-up uses the unique mapping attribute instead; all 75 enum cases pass with CRLF and the full 989-case generator suite passes after restoring LF. No assertions or production logic changed.

Local validation

• Release solution build: zero warnings or errors.
• Full generator: 989 passed; runtime: 5 passed; integration: 1 passed. Zero failures or skips in these suites.
• All three source samples pass. The consumer-skill validator, solution-path consistency check, CRLF-aware whitespace check, and independent read-only review pass.
• Available packaging: 224 passed, two infrastructure failures, zero skips. All 12 framework/language consumers, clean-package samples, package/API checks, usage checks and benchmark smoke passed. The shared example runs in the matrix, including genuine netstandard2.0/C#9 references.
• Local trimming and Native AOT stopped at MSBuild ILLink task-host creation (MSB4216 / SocketException 13). PowerShell and clang are absent; 49 PowerShell-dependent cases were not run locally. No tests or warnings were weakened. The successful exact-head CI below covers these remaining gates.

CI and review boundary

Verified published head: 532a3b484d9c480ae9af1cd3a2d59f16fbb0c085; tree: bb19214ff83fac1b678f249fc03afc0597d3885a matches the locally validated tree. CI #109 completed successfully on this exact head. Its checked-out merge tree also matches the validated head tree.

• Windows and Linux each passed 1,270 tests: 989 generator, 5 runtime, 275 packaging, and 1 integration; zero failures or skips. Both builds reported zero warnings/errors.
• All six Roslyn jobs (4.8.0, 4.14.0, 5.9.0 on both OS) each passed 989 tests, with zero failures or skips.
• Both full jobs ran the unfiltered packaging suite with LITEMAPPER_REQUIRE_NATIVE_AOT=1, covering trimming, Native AOT, and the PowerShell-dependent cases unavailable locally. Linux sample/benchmark smoke and both solution-path checks passed.
• Release-only jobs were skipped as expected for an ordinary PR.

This draft is ready for human review. No merge, release, tag, NuGet publication, or workflow dispatch was performed.

Nullable value-type intermediate traversal that fails earlier source-path resolution is outside this enum-lowering repair.

Prepared with OpenAI Codex.