fix: route llama.cpp log fallback to stderr instead of stdout

[andreinknv]

What

When the addon's llama.cpp log callback has no JS logger to delegate to —
either none was registered yet, or a registered logger threw — it falls
back to writing the log line directly. That fallback sent every
non-error line (warn / info / debug) to stdout; only error-level
lines went to stderr.

This routes all llama.cpp log output to stderr in both fallback sites
(addonCallJsLogCallback and addonLlamaCppLogCallback).

Why

For an embedder whose stdout is a data channel, the current behavior
corrupts output:

• A CLI that prints results to stdout gets llama.cpp's load: …,
  init: …, llama_*: … model-load chatter interleaved into those
  results — dozens of init: embeddings required but some input tokens were not marked as outputs -> overriding lines per run for an
  embeddings workload.
• An MCP / stdio server is worse: its stdout is a JSON-RPC stream, and
  non-JSON log lines on it are protocol corruption.

Library diagnostics belong on stderr regardless of level — that's the
standard contract, and what llama.cpp itself defaults to. The fallback
only fires when the JS logger is unavailable, so this changes nothing for
consumers with a working logger; it just makes the no-logger fallback
safe.

Change

Both fallback blocks previously did if (level == error) stderr; else stdout;. Since both branches now target stderr, I collapsed each to a
single fputs(…, stderr). If you'd prefer to keep the explicit level
check for readability, happy to restore it — the only intended behavior
change is non-error → stderr.

Testing

Verified with an embeddings workload that previously interleaved ~120
init: lines into stdout: stdout is now clean and the lines appear on
stderr.

Touches only llama/addon/globals/addonLog.cpp. No JS / API change.