QOL improvement with @PrimaryMappingSource instead of explicit source declaration

Nobody has claimed this yet.

### Use case

Sometimes, a mapping needs to borrow a field from another table.
```java
@Mapping(target = "borrowed", source = "tblB.borrowed")
EntityA toEntity(TblA tblA, TblB tblB)
```

But if those tables have many similar fields, then we'll get lots of errors
```
error: Several possible source properties for target property "id"
error: Several possible source properties for target property "name"
error: Several possible source properties for target property "description"
error: Several possible source properties for target property "createdAtUtc"
error: Several possible source properties for target property "updatedAtUtc"
```
which can be resolved by manually specifying the source of every conflicting property
```java
@Mapping(target = "id", source = "tblA.id")
@Mapping(target = "id", source = "tblA.name")
@Mapping(target = "id", source = "tblA.description")
@Mapping(target = "id", source = "tblA.createdAtUtc")
@Mapping(target = "id", source = "tblA.updatedAtUtc")
@Mapping(target = "borrowed", source = "tblB.borrowed")
EntityA toEntity(TblA tblA, TblB tblB)
```

My proposal is a new annotation called something like `@PrimaryMappingSource` which would be used like this
```java
@Mapping(target = "borrowed", source = "tblB.borrowed")
EntityA toEntity(
@PrimaryMappingSource TblA tblA,
TblB tblB
)
```

Which would have the effect of defaulting to tblA when there is any ambiguity in the source for a target property. This is similar to the `@org.springframework.context.annotation.Primary` annotation, which they explain like this:

>Indicates that a bean should be given preference when multiple candidates are qualified to autowire a single-valued dependency. If exactly one 'primary' bean exists among the candidates, it will be the autowired value.

### Generated Code

The generated code would be the same as manually specifying the source of all those ambiguous properties

### Possible workarounds

_No response_

### MapStruct Version

_No response_

The issue names no source files, tests, or entry points. Start by locating MapStruct's ambiguous source-property resolution and annotation-processing tests, then compare existing parameter-annotation conventions. Done should include a supported primary-source annotation, coverage for ambiguous properties, and generated mappings equivalent to explicit source declarations.

Written by the indexing model from the issue text.