v4.12.1

The first PyPI release: pip install mercury-composable. The v4.12.0 lock-step line
plus the AI-node demo functions below.

Added

• llm.chat demo function (examples/demo_app.py) - the AI node of the agent-orchestration
  experiment E0: a provider-neutral LLM adapter (Anthropic and Gemini backends behind one
  contract; select with llm.provider config / -Dllm.provider=... / params.provider)
  with structured output (schema -> JSON verdicts for graph decision routing;
  additionalProperties defaults closed), usage/stop_reason surfacing, provider-error
  mapping onto the envelope status, and the params.timeout_ms time-budget mapping.
  The SDKs are an optional extra (pip install "mercury-composable[llm]"); the package
  itself stays SDK-free (scope fence intact).
• llm.stream demo function - the STREAMING AI node: pulls the provider's real token
  stream (Anthropic messages.stream or Gemini generate_content_stream) and relays
  each token batch over the multi-shot reply contract, so a calling engine renders it
  progressively out its own HTTP edge as SSE. Terminal metadata carries model,
  stop_reason, usage and the trace/business correlation ids; provider errors fail the
  stream in-band. Live-proven end to end with real Gemini tokens (2026-08-31).
• PyPI publication metadata: SPDX license expression (PEP 639), trove classifiers,
  keywords, project URLs, and a constrained sdist (package sources, tests, examples
  and top-level docs only - repo-internal layers never ship).