Disambiguate nested record names that clash with a sibling field

[tannergooding]

Fixes #606.

A named nested record is legal in C even when a sibling field shares its name, but the two map to a nested type and a field of the same name in the same C# parent, which is a CS0102 clash. For example:

union GpuCaptureUnion {
    struct GpuCaptureParameters { int a; int b; } GpuCaptureParameters;
    int other;
};

previously emitted both public partial struct GpuCaptureParameters and public ... GpuCaptureParameters; in the same struct.

---

This does both of the things suggested on the issue:

• Auto-rename + warn. When a named nested record clashes with a sibling field, the type is renamed using the existing anonymous-record naming (e.g. _GpuCaptureParameters_e__Struct) so the declaration and every reference resolve to the same type. A warning is emitted once per rename pointing the user at the explicit options:

  Renamed nested type 'GpuCaptureParameters' to '_GpuCaptureParameters_e__Struct' to avoid a name clash with a field of the same name in 'GpuCaptureUnion'. Use '--remap-type ...' or '--remap-field ...' to control the naming explicitly.

• --remap-type / --remap-field. These disambiguate a type vs a field that share a name. They take precedence over the general --remap and are keyed by cursor kind, so resolving a field's type no longer picks up a --remap-field entry (and vice versa).

A couple of notable details:

• The clash is detected via the lexical parent (LexicalDeclContext), not the semantic one. In C a nested record is semantically promoted to the enclosing scope, yet it is still emitted nested based on where it lexically appears -- so the semantic parent would miss the clash for exactly the C headers that hit this.
• The type-reference path (value fields) resolves the leaf name from the type spelling rather than through GetRemappedCursorName, so the rename/override is re-applied there via a small shared helper to keep the declaration and reference in sync.

Verified the generated output compiles (no CS0102) for the default, --remap-type, and --remap-field cases. Added UnionFieldTypeNameClashTest covering all three. Full generator suite green (Failed: 0, 3724 passed) with zero golden churn.

Note

This PR description and the code changes were drafted with Copilot.