README polish: framework integration, exception pattern, fail-open note, async demo, trim Roadmap

[higagan]

Nice-to-haves from the UX review, bundled. None are demo-blocking; each strengthens the page for adopters. Pick off individually.

• Framework integration snippet — the Adaptive red-teaming fuzzer for modelfuzz scan #1 adoption question for the app-dev and manager personas: does @shield_tool compose with a LangChain @tool / OpenAI function-calling / MCP tool? One realistic wrap example.
• Exception-handling pattern — ~3 lines showing what the agent loop does when a block fires: catch ModelFuzzBlockError, feed "blocked" back to the model, vs. crash.
• Explicit fail-open-on-type note — the Limitations line lists the inspected types; state the consequence: a secret in bytes / int / a custom object passes silently. (Tracked deeper in the fail-open issue.)
• Show the async path — line 110 claims sync + async support but no example demonstrates async. Add a shielded async def.
• Show the stderr line the Try-It-Now run emits (ModelFuzz blocked tool call: …) so a careful reader isn't surprised when it appears.
• Trim the Roadmap (lines ~117–119) so "hosted dashboard" doesn't interrupt the argument between Limitations and the scanner — reads less like selling futures.
• Version/maturity signal — a one-line note that it's pre-1.0 (0.3.x) sets expectations honestly.

Done when

• Each item either addressed or consciously declined