Add an xunit compile-back gate over the fixture green set

[richlander]

What

Promotes the compile-back oracle (decompile → recompile → compare IL) from a
console-only harness mode into a durable CI regression gate.

Why

Compile-back is the only rail that verifies semantic fidelity — that decompiled
C#, recompiled, produces the same IL. A body that parses, binds, and reads
plausibly but recompiles to a different opcode stream changed the program, the
worst failure class, invisible to --compile-check and --grade-source. Until
now it only ran when someone manually invoked --compile-back; nothing stopped a
regression from landing.

How

• CompileBack.Evaluate: a non-printing, structured-result entry point that
  shares all of the skeleton-emission and opcode-comparison machinery with the
  existing console Run, so the two paths cannot drift. Run is untouched.
• The test project links the single CompileBack.cs source file (rather than
  a project reference on the harness Exe, which would pull in a second entry point)
  and adds the Roslyn package it needs.
• CompileBackGateTests asserts two properties over CfgSampleClass:
  • NoNewOpcodeDiffsBeyondKnownDocket — fails if a method newly recompiles to a
    different opcode stream beyond the documented KnownDiffs docket. Catches a fix
    in one method silently degrading another.
  • PinnedFixesStayOpcodeExact — CheckedAdd (Add the compile-back oracle and fix dropped overflow checks #604), UnsignedShift (Drop the redundant width mask on a shift count #606), and
    Shadowed (Qualify shadowed instance field loads with this. #607) must stay opcode-exact.

KnownDiffs is the open docket and shrinks as fixes land. StaleFieldRead stays a
tracked defect (#605).

Verification

• Both gate tests green on current main.
• Full ILInspector.Decompiler.Tests suite green (428 passing).

Part of the compile-back oracle docket (#604). Related: #605.

Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com