Add std/net DNS lookup helpers

[larimonious]

Summary

Adds the DD-046 DNS slice for std/net:

• dns_lookup(name, record_type?, opts?) -> Result<Array<Map>, String>
  • Defaults record_type to A.
  • Supports data-bearing DNS lookup records: A, AAAA, ANAME, CAA, CDNSKEY, CDS, CNAME, CSYNC, DNSKEY, DS, HINFO, HTTPS, KEY, MX, NAPTR, NS, NSEC, NSEC3, NSEC3PARAM, NULL, OPENPGPKEY, PTR, RRSIG, SIG, SOA, SRV, SSHFP, SVCB, TLSA, TXT.
  • Rejects operational/meta record types like ANY, AXFR, IXFR, OPT, TSIG, and ZERO rather than treating them as normal app-level lookups.
  • Uses the system resolver only; custom nameserver support remains deferred because it can bypass DNS policy.
  • Returns record maps with actual returned type, name, value, and ttl.
• dns_reverse(ip, opts?) -> Result<Array<String>, String>
  • Returns all PTR names for IPv4/IPv6 reverse lookup.
• Keeps no-answer DNS responses as Ok([]) and invalid input / resolver setup / transport failures as Err(String).
• Updates generated stdlib docs, AI guide, DD-046, typechecker signatures, and examples/std_net_dns.tnt.

Test Plan

• cargo fmt -- --check
• git diff --check
• cargo check --profile dev-release
• cargo test --lib stdlib::net::tests -- --nocapture
• cargo test --test std_net_tests -- --nocapture
• cargo test --test type_checker_tests std_net_phase1 -- --nocapture
• cargo build --profile dev-release && ./target/dev-release/ntnt docs --generate
• ./target/dev-release/ntnt validate examples/
• ./target/dev-release/ntnt lint examples/std_net_dns.tnt
• cargo test
• NTNT_NET_EXTERNAL_DNS_TESTS=1 cargo test --test std_net_tests dns_lookup_external_smoke_is_opt_in -- --nocapture

Notes

External DNS smoke coverage stays opt-in so default CI is deterministic and does not depend on public DNS/network availability.

[greptile-apps]

Greptile Summary

This PR adds DNS lookup helpers to std/net. The main changes are:

• Adds dns_lookup and dns_reverse runtime functions using the system resolver.
• Supports a broad set of data-bearing DNS record types and rejects operational/meta types.
• Returns DNS records as maps with actual type, name, value, and TTL.
• Adds typechecker signatures, stdlib docs, design-doc updates, examples, and tests.

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed code.
• The latest example change keeps public DNS calls behind an explicit environment opt-in.
• The earlier top-level DNS example behavior is no longer present.

_{Reviews (4): Last reviewed commit: "Make std net DNS example opt-in" | Re-trigger Greptile}