fix: correct the recursion available header flag to the rfc 1035 value

[bdraco]

Summary

_FLAGS_RA has been 0x8000 since 2009, which is the QR bit. RFC 1035 section 4.1.1 puts Recursion Available at bit 7, 0x0080.

Details

• The constant is defined and never used (mDNS ignores RA per RFC 6762 section 18.6), so no packet ever carried the wrong value; this only matters if the constant is ever imported or used.
• Every other flag in the block matches RFC 1035 section 4.1.1.

Test plan

• full test suite: 518 passed
• pre-commit clean

[github-actions]

Thanks for contributing! A quick note: this project is in the process of moving from LGPL-2.1 to Apache 2.0, see #1835.

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[codspeed-hq]

Merging this PR will improve performance by 8.19%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 1 improved benchmark
✅ 23 untouched benchmarks

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	test_sending_packets	112.7 µs	104.1 µs	+8.19%

Tip

Curious why performance improved? Comment @codspeedbot explain why performance improved on this PR, or directly use the CodSpeed MCP with your agent.

---

_{Comparing fix_ra_flag (e1377d1) with master (d5f4102)}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.87%. Comparing base (d5f4102) to head (e1377d1).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1893   +/-   ##
=======================================
  Coverage   99.87%   99.87%           
=======================================
  Files          33       33           
  Lines        3911     3911           
  Branches      568      568           
=======================================
  Hits         3906     3906           
  Misses          3        3           
  Partials        2        2           

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