[BUG] runtime segfault at non-canonical address 0xFFFFFFFFFFFFFFFF kills the CLI mid-session (Windows, v2.1.239)

### Preflight Checklist

- [x] I have searched [existing issues](https://github.com/anthropics/claude-code/issues?q=is%3Aissue%20state%3Aopen%20label%3Abug) and this hasn't been reported yet
- [x] This is a single bug report (please file separate reports for different bugs)
- [x] I am using the latest version of Claude Code

### What's Wrong?

After roughly 2 hours of an interactive session, `claude.exe` died with a Bun panic and dropped
back to the PowerShell prompt. The session was lost. Two other concurrent `claude` processes on
the same machine were unaffected and kept running.

```
Bun v1.4.0 (fdb5e06cc) Windows x64
Windows v10.26200
CPU: sse42 avx avx2
Args: "C:\Users\\.local\bin\claude.exe" "--dangerously-skip-permissions"

Elapsed: 7147917ms | User: 24812ms | Sys: 21437ms
RSS: 0.34 GB | Peak: 0.62 GB | Commit: 0.72 GB | Faults: 345015 | Machine: 33.58 GB

panic(main thread): Segmentation fault at address 0xFFFFFFFFFFFFFFFF
```

Bun crash report: https://bun.report/1.4.0/w_1fdb5e06mgggkIuhogCijwr3D_os177C________A2DD

### What Should Happen?

it doesn't crash ;)

### Error Messages/Logs

```shell

```

### Steps to Reproduce

No deterministic repro — the crash happened once, mid-session, and has not recurred.
Reporting it for the crash-report URL rather than as a reproducible defect.

Conditions at the time:
1. Run `claude --dangerously-skip-permissions` in a large repo (Windows, native build).
2. Keep an interactive session alive for ~2 hours (Elapsed: 7147917ms).
3. Two other `claude` processes were running concurrently against the same working tree;
both survived, only this one died.
4. The crash occurred while the agent wrote a small Markdown file and read a sibling
index file (auto-memory write). Small payload, no unusual tool.

### Claude Model

Opus

### Is this a regression?

Yes, this worked in a previous version

### Last Working Version

_No response_

### Claude Code Version

2.1.139

### Platform

Anthropic API

### Operating System

Windows

### Terminal/Shell

Windows Terminal

### Additional Information

Start with the linked Bun crash report and the reported Bun 1.4.0, Windows x64, and Claude Code versions. Compare the long interactive session, concurrent processes, and auto-memory Markdown write conditions, but note that the report has no deterministic reproduction or additional logs. Done means the cause of the native segfault is identified and the CLI no longer terminates mid-session.

Written by the indexing model from the issue text.