fix: prioritize explicit root-source converters

[AGiorgetti]

Change

Fixes #23.

• Resolve an explicit omitted-Source converter against the root object before automatic source-member matching in new-object, constructor-bound, and existing-target mappings.
• Keep unrelated same-name/ambiguous members and patch getters from overriding that selection. Root parameters retain entry-level null guards.
• Add 18 executable regressions plus Basic/clean-package consumer coverage and synchronized usage, changelog, and control documents.
• Preserve existing converter diagnostics, explicit source paths, public signatures, and exception behavior.

Verification

• Test-first: 16 initial cases on unchanged develop produced 11 failures and five controls, zero skips. The Basic sample independently failed with LITEMAPPER1002/CS8795. A nullable-root patch extension then reproduced CS1061 before the root-parameter member-guard correction; the explicit-path control passed.
• Final local generator suite: 652 passed, zero failed/skipped, including all 18 new cases. Runtime 5/5, integration 1/1, and all three source sample smoke runs pass.
• Release solution build has zero warnings/errors. Generated Basic source was inspected: BuildDisplayName(source) receives the root once.
• Available packaging suite: 220 passed, two infrastructure failures, zero skips, including all 12 consumer framework/language combinations, clean package-backed samples, package/API checks, usage validation, and benchmark smoke.
• Trimming and Native AOT are locally blocked at ILLink ComputeManagedAssemblies task-host creation (MSB4216 / Unix-domain SocketException 13). PowerShell and clang are absent; 49 PowerShell-dependent cases were not run locally. No local trimming/AOT success is claimed. An earlier packaging attempt was interrupted by a tool approval cancellation and is not counted; the completed retry is reported above.
• Independent read-only review found no scoped correctness blocker. CRLF-aware diff and consumer-skill validation pass.
• Remote tree 6a7d60b8ed091e0f83913379ecaeb53c10862a79 matches the tested/staged local tree. Exact-head ordinary PR CI passed for 1e274b9: Windows and Linux each passed 929 tests (652 generator, 271 packaging, five runtime, one integration), zero failures/skips; all six Roslyn 4.8.0/4.14.0/5.9.0 OS jobs passed 652 tests each. The full packaging suite covers PowerShell, trimming and Native AOT checks unavailable locally. Source samples, benchmark smoke, and solution-path validation passed. Release pack/artifact-verification/publication jobs were skipped as expected for this ordinary PR.

Retained as draft for human review. No issue comments/claim writes, merge, release, tag push, package publication, or workflow dispatch was performed. Other issues remain outside this fix.

Prepared with OpenAI Codex.