cs2gs: fail closed and recover after ILVerify crashes

[DavidObando]

Summary

• classify abnormal ILVerify exits as Incomplete, never as ordinary verifier findings
• rerun with verbose-identified crashing members excluded, preserving pre-crash and recovering post-crash findings
• emit a distinct IlVerifyIncomplete artifact alongside recovered findings

Tests

• deterministic crash-before/after, zero-output crash, verifier fail/pass, and stage artifact coverage
• IlVerifyStageTests: 16 passed
• full Cs2Gs.Tests: 1,719 passed

Deferred

• The malformed while true compiler emission remains tracked separately by gsc: infinite while-true loop emission crashes dotnet-ilverify #2747.
• If a tool crash emits no Verifying marker, method recovery is impossible; the run still fails closed with explicit incompleteness.

Fixes #2748