[CI] Roslyn csc exits with SIGSEGV (MSB6006 code 139)

## Additional info to collect

Copilot: please collect the following information for each occurrence of this issue:
* Any crash reports for the crashed process. Upload them to the issue, or if that's not possible, create gists.
* We have minidumps enabled for some parts of the build (DOTNET_DbgMiniDumpName); collect whether the crash created a minidump or not, and if so, a link to the artifact where the minidump was uploaded.

## Failure signature

The Roslyn compiler process crashes during `CoreCompile` on macOS CI workers:

```text
Microsoft.CSharp.Core.targets(97,5): error MSB6006: "csc" exited with code 139.
```

Exit 139 is SIGSEGV. No matching crash report was published for either failed job, so the native crashing stack is unavailable.

## Confirmed occurrences

| PR / head | Build attempt / job | Configuration | Worker |
|---|---|---|---|
| #26280 / `4a9126ef44460e731dfe277e1c125b193eb713ce` | [14753725 attempt 1 — dotnettests_maccatalyst / Run tests](https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=14753725&view=logs&j=67959b4e-f34b-5667-75bd-2d99b3b69779&t=c8f692bc-2c0b-592c-1f0d-1a2812ad5f0b) | `PInvokeWrapperGenerator(MacCatalyst, maccatalyst-x64)`; MySimpleApp CoreCompile | `AcesShared 103` |
| #26285 / `519c5da35aaa8791a48b23a42c0012ad84c38715` | [14754963 attempt 1 — linker_maccatalyst / Run tests](https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=14754963&view=logs&j=dc49fd7f-b39a-580f-b9a6-2209b7abaf55&t=1df1eb8b-f9c9-5620-65f1-63d8d5893e86) | dont-link Mac Catalyst Release, PrepareAssemblies/MonoVM/Dynamic Registrar | `AcesShared 95` |

**Total:** 2 build/job occurrences across 2 monitored PRs.

First observed: 2026-07-24 02:41 UTC. Latest observed: 2026-07-24 03:35 UTC.

## Classification and impact

This is a CI infrastructure/tool-process crash rather than a repository diagnostic:

- Both jobs use the same .NET SDK/Roslyn build (`10.0.400-preview.0.26371.110`).
- The crashes occur in unrelated PRs and unrelated test projects.
- Binlogs contain only `MSB6006`; #26280 failed after a 639 ms Csc task and #26285 after a 436 ms Csc task.
- The CrashReports containers contain no report for either affected job.

The crash aborts otherwise unrelated test configurations.

## Investigation performed

- Inspected TestSummary and HtmlReport artifacts.
- Analyzed both failing binlogs and confirmed the error originates in the `Csc` task under `CoreCompile`.
- Searched all open and closed dotnet/macios issues using the exact MSB6006 text, exit code, test names, and Roslyn/compiler terms; no matching issue existed.

## Retry status

- #26280 simulator_tests attempt 2 was started by an external actor and is currently rerunning dotnettests_maccatalyst.
- #26285 build 14754963 remains active; no retry is permitted until it is terminal.

Azure artifacts and logs have limited retention.

Start with the linked build attempts and their CoreCompile/Csc entries, then inspect the binlogs, TestSummary, HtmlReport, and CrashReports artifacts. Done means documenting whether each occurrence produced a crash report or minidump and linking any available artifacts, while recording when the relevant artifacts are absent.

Written by the indexing model from the issue text.