fix(csharp): explicit interface implementations are public, not private

[zichen0116]

Closes #2164

Problem

Since #1745, cross-file resolution declines candidates whose visibility is
private (they are treated as file-local) — and the C# extractor defaulted
every modifier-less member to private. That default is wrong for explicit
interface implementations: void IOrderService.CancelOrder(int id) cannot
carry an access modifier in C# (CS0106), but it is reachable through the
interface, so it must not be treated as file-local.

The two other cases from #2164 (interface members, top-level types) were
already fixed by #2220; this covers the remaining one.

Fix

In the C# getVisibility, return public when the member node carries an
explicit_interface_specifier child (methods, properties, indexers, events —
all share this node shape):

// An explicit interface implementation (`void IFoo.Bar()`) cannot carry
// an access modifier; it is reachable through the interface, so it is
// public even though its class body declares it (#2164).
if (child?.type === 'explicit_interface_specifier') return 'public';

Verification

New regression test __tests__/csharp-interface-visibility.test.ts (real
indexing through the full CodeGraph pipeline):

• interface member without a modifier → visibility = public (guard for fix(csharp): namespaces, usings and nested types follow the language's scopes #2220)
• the issue's original repro (interface in one file, called through a field,
  a parameter, and a primary-constructor parameter in another file) → 3
  calls edges to IOrderService::CancelOrder (guard for fix(csharp): namespaces, usings and nested types follow the language's scopes #2220)
• before this fix: the explicit implementation was extracted as
  visibility = private, and a cross-file call to it produced no calls
  edge; after: visibility = public and the call resolves. The new test
  fails on the pre-fix tree and passes with the fix.

Also ran: all 5 C# suites (11 tests) pass, __tests__/extraction.test.ts
(691 tests) passes, tsc --noEmit clean. Added a CHANGELOG.md entry under
[Unreleased] → ### Fixes per the repo's changelog rules.