Phantom: Arbitrary file write and decode-bomb DoS via unconfined MCP tool paths

Summary

### Impact

In Phantom <= 1.3.0, when `PHANTOM_OUTPUT_DIR` was unset (the default), the MCP tools accepted arbitrary absolute output paths with no confinement. Anything able to send tool calls (e.g. an AI agent driving the MCP interface) could **write or overwrite arbitrary files** the process user can write — including shell startup files (`~/.zshrc`) or a Reaper `__startup.lua`, which is effectively local code execution on a developer workstation.

Separately, the stem-separation and render paths decoded input audio with no size/duration cap (the analysis path was already guarded). A small, highly compressed FLAC/OGG could expand to multi-gigabyte PCM, causing memory-exhaustion DoS, and widened exposure to decoder bugs including libsndfile CVE-2026-37555.

### Patches Fixed in **1.3.1**: - File writes are always confined to `PHANTOM_OUTPUT_DIR` (default `~/.phantom/output`); symlinks resolved and re-verified on the final path. - Decode/duration/size guards mirrored onto the separation and render paths (plus ffmpeg `-max_alloc`/`-t`/`-fs`). - Atomic `O_CREAT|O_EXCL` output creation in reference matching and symlink-TOCTOU hardening on confined input reads.

### Workarounds Set `PHANTOM_OUTPUT_DIR` (and optionally `PHANTOM_AUDIO_DIR`) to dedicated directories before starting the server.

### Credit Found during an internal security audit.