Display the IL 'tail.' prefix in C# output

[siegfriedpammer]

Fixes #3817.

F# emits tail calls pervasively, but the IL tail. prefix was dropped entirely at the C# output stage, so the information never reached the decompiled text. This surfaces it as an inline /*tail.*/ marker before the call:

public static int Caller(int x)
{
    return /*tail.*/Callee(x);
}

The flag (CallInstruction.IsTail) was already tracked end-to-end through the IL pipeline; it was simply discarded when the call became a C# invocation. The implementation mirrors the existing constrained.-prefix comment already emitted in CallBuilder.

Notes

• Always on, no setting/toggle.
• No existing fixtures contain a real tail. opcode (the "tail" hits in the F# fixtures are get_TailOrNull() substrings), so the change has zero churn on the rest of the suite.
• calli (CallIndirect) does not model IsTail, so tail-prefixed indirect calls are not marked (rare; out of scope).

Test

New ILPretty fixture TailCall.il/TailCall.cs exercising a tail. call followed by ret.

---

🤖 This PR was prepared by Claude Code (claude-opus-4-8) on behalf of @siegfriedpammer.