docs: add runnable AsyncDualMDNSResolver example

[aiolibsbot]

What

Add examples/dual.py, a runnable example that resolves a .local host through
an aiohttp ClientSession using AsyncDualMDNSResolver.

Why

AsyncDualMDNSResolver is documented as a first-class option in the README,
docs/api.rst, and docs/index.rst, but examples/ only demonstrated the
plain AsyncMDNSResolver. The dual resolver is the one most useful in real
deployments (mDNS and unicast DNS concurrently, first success wins), yet
users had no copy-pasteable runnable form of it.

How

• Mirrors the structure of examples/run.py so the two are directly comparable.
• Uses the async with resolver pattern (resolver isn't owned by TCPConnector),
  matching the convention swept in across all snippets by docs: use async context manager in examples and quick starts #92.
• Module docstring captures the practical "when to choose dual" rationale from
  the README so the example is self-documenting.

Testing

• python -m py_compile examples/dual.py (local env can't run pytest — no zeroconf).
• examples/ is not imported by the test suite or referenced from docs, so this is
  purely additive.

🤖 Generated with Claude Code

---

Quality Report

Changes: 2 files changed, 34 insertions(+)

Code scan: 1 issue(s) found

• examples/dual.py:28 — debug print statement

Tests: failed (FAILED)

Branch hygiene: clean

Generated by Kōan post-mission quality pipeline

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (5a01499) to head (d8c6e1e).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

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  Coverage   100.00%   100.00%           
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  Files            6         6           
  Lines          521       521           
  Branches        23        23           
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  Hits           521       521           

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