ds4-server Bus error (SIGBUS) in vocab_token_is_literal_special during generation after successful 34K-token distributed prefill

**Title: ds4-server Bus error (SIGBUS) in vocab_token_is_literal_special during generation after successful 34K-token distributed prefill**

## Summary

`ds4-server` crashes with a Bus error (SIGBUS / KERN_MEMORY_ERROR) at the start of generation, immediately after a successful 34,384-token distributed prefill. The prefill itself completes without errors at ~152 t/s (225 seconds). The crash happens when the server tries to convert the first generated token ID back to text via the vocabulary table.

The same long prompt works fine via the CLI (`./ds4 --prompt-file`), so this appears specific to the `ds4-server` binary's generation/vocab path.

## Crash Details

```
Type: EXC_BAD_ACCESS | KERN_MEMORY_ERROR at 0x0000007000233f35
Termination: Bus error: 10

Faulting stack:
ds4-server vocab_token_is_literal_special
ds4-server ds4_token_text
ds4-server generate_job_inner
ds4-server generate_job
ds4-server worker_main
libsystem_pthread.dylib _pthread_start
```

Bus error (not SIGSEGV) suggests a truncated/corrupted mmap region or misaligned access — possibly the vocab mmap became invalid during the 225-second prefill wait.

## Environment

- ds4 main branch (commit ~8db89fe / cc231cb era), macOS 26.6.2, arm64, Metal backend
- 2× MacBook Pro M1 Max 64GB, Thunderbolt 4 bridge, static IPs
- Model: DeepSeek-V4-Flash-0731 DS4-Quality128 (95.76 GiB, SHA verified)
- Split: coordinator layers 0:22 (A) / worker layers 23:output (B)
- ctx=262144, prefill-chunk=1024, dist-activation-bits=8
- Prompt: 34,384 tokens (ZCode agent system prompt + tools)

## Reproduction

1. Start worker on machine B: `./ds4 --metal -m model.gguf --role worker --layers 23:output --coordinator 5555 -c 262144 --prefill-chunk 1024`
2. Start API server on machine A: `./ds4-server --metal -m model.gguf --role coordinator --layers 0:22 --listen 5555 -c 262144 --prefill-chunk 1024 --dist-activation-bits 8 --host 0.0.0.0 --port 8002`
3. Send a POST to `/v1/chat/completions` with a ~34K token message
4. Prefill completes at 100% (~225s), then ds4-server crashes with Bus error

## Key Observations

- The **prefill completes successfully** (100%, 225s, 152 t/s avg) — the distributed pipeline works
- The crash is in **generation/vocab lookup**, not in compute
- CLI mode (`./ds4 --prompt-file`) with 51K tokens **works fine** on the same setup
- Short prompts (<20K) via API **work fine**
- The Bus error (not SIGSEGV) points to an mmap/truncation issue rather than a wild pointer

## Guess

The vocabulary/token table mmap may become invalid during the long prefill wait — possibly USB SSD page eviction, mmap truncation under memory pressure, or a ds4-server-specific mmap handling difference from the CLI path.

Happy to test a fix or provide the full .ips crash report.