docs: add notes on c-ares integration extracted from PR #75

[manugarg]

Extracted useful design notes and patterns about c-ares integration from the unmerged PR #75 (Google backporting), which is not a priority but contains a well-structured c-ares integration worth learning from.

Saved as docs/c-ares-integration-notes.md. Covers:

• resolver abstraction (none / getaddrinfo / c-ares function pointer + runtime selection)
• compile-time gating (HAVE_C_ARES, dummy fallbacks, pkg-config auto-detection)
• lifecycle ordering and the partial-init cleanup pattern
• synchronous select() wrapper over c-ares's async API
• shared result collector keeping output semantics identical across backends
• new public API, pactester CLI options, and DNS test patterns
• porting caveats (old c-ares API, known memory bugs, SpiderMonkey vs current QuickJS layout)

Docs-only change; doxygen generation is unaffected (it runs in a temp dir with only pacparser.h).

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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