feat(orm): add explicit partial repository updates

[GaspardKirira]

Summary

Allow selected entity fields to be updated without requiring a complete Mapper<T>::toUpdateFields() result.

Current behavior

updateById() always updates every field returned by toUpdateFields().

HTTP PATCH-style operations require manual SQL or a custom repository method.

Expected behavior

Repositories must support an explicit field list for partial updates while preserving mapper-based full updates.

Scope

• Add a partial update operation.
• Accept validated FieldValues.
• Reject empty updates.
• Validate field identifiers.
• Bind every value safely.
• Support configurable primary-key columns.
• Support a transaction-bound repository.
• Add SQLite integration tests.
• Preserve updateById().

Suggested files

• modules/orm/include/vix/orm/Repository.hpp
• modules/orm/tests/repository_partial_update_test.cpp

Acceptance criteria

• One field can be updated.
• Multiple fields can be updated.
• Unspecified columns remain unchanged.
• Empty field lists fail clearly.
• Invalid identifiers are rejected.
• Values are bound through prepared statements.
• Existing full-update behavior remains unchanged.

Non-goals

This issue does not include:

• JSON Patch parsing;
• automatic dirty tracking;
• relationship updates;
• schema validation.

Suggested labels

• scope:orm
• type:feature
• tests
• good first issue
• status:accepted