fix: discover attributed partial mappers once

[AGiorgetti]

Change

Fixes #24.

• Plan a merged partial mapper only from the declaration owning its first LiteMapper marker. Other attributed parts still contribute members and UseMapper registrations.
• Preserve existing fatal duplicate-marker diagnostics and incremental candidate identity. No public API or specification semantics change.
• Add 24 executable regressions plus Basic/clean-package sample coverage and synchronized usage, changelog, and control documents.

Verification

• Tests-first on unchanged develop: 23 failures and one passing control, zero skips. The Basic sample separately reproduced duplicate-hint CS8785, discarding all three mappers' generated implementations and producing CS8795.
• Final local generator suite: 676 passed, zero failed/skipped, including all 24 new cases. Runtime 5/5, integration 1/1, and all three source sample smoke runs pass.
• Release solution build: zero warnings/errors. Generated Basic source was inspected: exactly one file per mapper, with the existing mapping body intact; its rebuild/run passes.
• 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 initial attribute-targeted-provider prototype recreated identities in two existing incremental tests. The final narrow owner filter retains the established provider; no existing tests were modified. New tests require one plan per mapper per run, stable hint/source content, and cached unchanged/reordered emissions.
• Independent read-only review found no scoped blocker. CRLF-aware diff validation passes.
• Remote tree d6fba1646a38562f967e0e3031876f27d0888da3 matches the tested/staged local tree. Exact-head ordinary PR CI passed for 76ce90c: Windows and Linux each passed 953 tests (676 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 676 tests each. Full packaging covers the PowerShell, trimming, and Native AOT checks unavailable locally. Both solution builds have zero warnings/errors; 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.