Bind bootstrap DNS lookups to -S source address (#1)

[karl82]

Motivation:

PR #196 added the -S flag to bind HTTPS connections to a source address, enabling policy-based routing. However, bootstrap DNS queries (used to resolve DoH server hostnames like "dns.google") were not bound to the source address.

This caused two issues:

1. Privacy leak: Bootstrap DNS queries go via default route (local ISP), exposing which DoH server you're using
2. Routing mismatch: HTTPS connection routes via VPN but may fail if resolved IP is unreachable from VPN

Implementation:

• Bind bootstrap DNS queries using ares_set_local_ip4() and ares_set_local_ip6() from c-ares
• Validate address family matches proxy mode (-4/-6), warn on mismatch
• Warn on invalid address literals
• Robot Framework tests for source binding and validation warnings
• Docker-based test infrastructure for CI/CD and macOS development

Example Usage:

https_dns_proxy -S 192.168.12.1 -b 1.1.1.1,8.8.8.8 -r https://dns.google/dns-query

With PBR rules routing traffic from source 192.168.12.1 via VPN:

# Route DoH HTTPS (port 443) via VPN
config policy
	option name 'DoH WA via wg_wa'
	option interface 'wg_wa'
	option chain 'output'
	option proto 'tcp'
	option src_addr '192.168.12.1'
	option dest_port '443'

# Route bootstrap DNS (port 53) via VPN
config policy
	option name 'Bootstrap DNS WA via wg_wa'
	option interface 'wg_wa'
	option chain 'output'
	option proto 'udp'
	option src_addr '192.168.12.1'
	option dest_port '53'
	option dest_addr '1.1.1.1 8.8.8.8'

Both rules now match because -S binds both HTTPS and bootstrap DNS to the same source address.

Verification:

Bootstrap DNS bound to source address:

[I] dns_poller.c:163 Using source address: 192.168.12.1
[I] dns_poller.c:208 Received new DNS server IP: 142.250.80.110 for dns.google

Warning on address family mismatch:

[W] dns_poller.c:133 Bootstrap source address '::1' is IPv6, but IPv4-only mode is set

Warning on invalid address:

[W] dns_poller.c:141 Bootstrap source address 'not-an-ip' is not a valid IP literal

Files Modified:

• src/dns_poller.c: Added set_bootstrap_source_addr() function
• src/dns_poller.h: Added source_addr parameter to poller init
• src/main.c: Pass source_addr to dns_poller
• src/options.c: Fix format string type
• tests/robot/functional_tests.robot: Source binding and validation tests
• tests/docker/Dockerfile: Test image with valgrind and ctest integration
• tests/docker/run_all_tests.sh: Simplified test runner using Dockerfile CMD
• CMakeLists.txt: Fix robot test WORKING_DIRECTORY, add distclean target
• README.md: Update Docker test documentation
• .gitignore: Add build/ directory