Taint propagation: `String.Trim()` (first instance-method target)

[JVBotelho]

Summary

Add native taint propagation for String.Trim() — the first instance-method propagation
target (every existing/other-queued target so far is static). Establishes the pattern for
TrimStart(), TrimEnd(), ToUpperInvariant(), ToLowerInvariant() as follow-up issues.

Background

ADR 006
ships v1 taint propagation for String.Concat(string, string) only, a static method. Trim()
is an instance method with zero declared parameters — the only operand to forward is this.

What changes

• RaspProfiler::IsV1PropagationTarget (src/Rasp.Native.Profiler/src/RaspProfiler.cpp:443-483):
  add a signature match for Trim — 0 declared params, System.String return type, and
  the calling-convention byte must test the IMAGE_CEE_CS_CALLCONV_HASTHIS bit (this is the
  first target where that bit needs checking; Concat is static and never sets it).
• RaspTaintSensor (src/Rasp.Core/Context/RaspTaintSensor.cs:85-96): add a
  PropagateTaint(string? result, string? operand) two-argument overload — marks result
  tainted if operand (the original this) was tainted.
• RaspProfiler::DoJITCompilationStarted (RaspProfiler.cpp:606-648): resolve the new
  overload's MemberRef and call RewritePropagationProbe with operand count 1 (just
  ldarg.0, i.e. this).

Acceptance criteria

• Trim() on a tainted string produces a tainted result.
• Trim() on an untainted string produces an untainted result.
• Existing Concat propagation is unaffected.
• Unit test added in Rasp.Core.Tests/Context/RaspTaintSensorTests.cs.
• PR description notes that TrimStart(), TrimEnd(), ToUpperInvariant(), and ToLowerInvariant() can each reuse this exact pattern as their own follow-up issue.