csharp: emit property nodes for record positional parameters

[pbednarcik]

Follow-up to the #478 review comment: positional record parameters — the synthesized public properties that are a record's defining members — emitted nothing. public record Medal(int Id, string Motto) produced the Medal type node with no Medal.Id or Medal.Motto.

Unlike the #478 gap, there is no declaration node for the member walk to find: the properties exist only in the compiler's synthesis. So the record's container emission now fabricates one property node per positional parameter, with the declared parameter type riding as field_type, public visibility, a positional: true provenance marker, and the usual defines/member_of edges. One grammar wrinkle handled along the way: a record's parameter_list carries no field name (unlike method parameters), so it is found by child-type scan.

Container emission precedes the body's member matches in tree order, so an explicit redeclaration of a positional property (legal C# — it replaces the synthesized one) deduplicates to a single node for the same logical member. Covers record and record struct.

Tests: property nodes for both positional parameters plus the record-struct case (kind, receiver, declared type, visibility, member_of edge), coexistence with explicit body members, and the redeclaration dedupe. Written first, watched fail. internal/parser/languages fully green; resolver and indexer at their pre-existing Windows baselines.

Validated live on a counted C# fixture repo: all six authored positional properties emit (across record, record struct, and a record with mixed positional + explicit members), pinned as counted battery cells, green twice.

Bumps the csharp extractor version 6→7 to restage .cs files.