Extend --with-conditional to multi-file output

[tannergooding]

Follow-up to #772, which added --with-conditional for single-file output. This extends the same option to multi-file output: each generated C# file is wrapped in a leading #if <symbol> and trailing #endif using the same single global symbol.

// StructOne.cs
#if VERSION_2_OR_NEWER

namespace Demo.Ns
{
    public partial struct StructOne
    {
        public int x;
    }
}
#endif

Same two choke points as single-file, just in the multi-file writer (CloseOutputBuilder): the #if goes after the header/before the usings, the #endif after the namespace close. Empty-contents builders are already skipped before the writer runs, so every emitted file gets a matched pair.

---

XML output and per-file (varying) symbols remain out of scope, same as #772 — a per-file symbol wants a remap and is a separate design. The golden-file harness is single-stream, so the multi-file test drives the generator directly with a per-path capturing stream factory (ConditionalMultiFileOutputTest).

Relates to #365