feat(namesys): propagate DNSLink TXT TTL

[lidel]

Problem

boxo's DNS resolver discards the TTL of DNSLink TXT records and returns 0. The gateway sets Cache-Control: max-age for mutable paths only when the TTL is non-zero, so DNSLink (/ipns/<dnslink-host>) responses go out with no max-age and fall back to a static value, even though the TXT record states exactly how long the mapping is valid (#329). Recursive names made it worse: a name resolved through several records was cached for only its final hop's TTL, ignoring shorter-lived links earlier in the chain.

Fix

• Carry the TXT record TTL into the resolved result (AsyncResult.TTL / Result.TTL), which the gateway already turns into Cache-Control: max-age.
• Add NewDNSResolverWithTTL, WithDNSResolverWithTTL, and LookupTXTWithTTLFunc for a TTL-aware lookup. WithDNSResolver auto-detects a resolver that implements madns.TXTWithTTLResolver and uses it.
• Cap a resolved name's TTL to its shortest hop, so a DNSLink pointing at an IPNS name, or a recursive IPNS chain, is not cached past its earliest-expiring link. Hops with an unknown TTL (0) are ignored.

NewDNSResolver and LookupTXTFunc keep their signatures and report an unknown TTL (0), so existing callers are unchanged; the new behavior stays inert until the resolver passed to WithDNSResolver actually reports TTLs.

Upstream PRs to land first

Required before this PR can build and merge:

• feat: add LookupTXTWithTTL to Resolver multiformats/go-multiaddr-dns#75 adds madns.TXTWithTTLResolver and routes LookupTXTWithTTL. go.mod pins its commit here; re-pin to a tagged release once it merges.

Needed only to switch DNSLink Cache-Control on end-to-end (a separate follow-up boxo PR, not this one):

• feat: add LookupTXTWithTTL to expose TXT record TTL libp2p/go-doh-resolver#33 lets the DoH resolver report the TXT TTL.
• a boxo go.mod bump of both deps, with no code change, since WithDNSResolver auto-detects the capability.

Part of #329. ETag for resolved paths (the issue's other half) is out of scope.

[codecov]

Codecov Report

❌ Patch coverage is 79.16667% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 63.39%. Comparing base (fc2837e) to head (4a82052).

Files with missing lines	Patch %	Lines
namesys/namesys.go	37.50%	4 Missing and 1 partial ⚠️

@@           Coverage Diff           @@
##             main    #1167   +/-   ##
=======================================
  Coverage   63.39%   63.39%           
=======================================
  Files         268      268           
  Lines       26967    26987   +20     
=======================================
+ Hits        17096    17109   +13     
- Misses       8147     8153    +6     
- Partials     1724     1725    +1     

Files with missing lines	Coverage Δ
namesys/dns_resolver.go	73.68% <100.00%> (+1.46%)	⬆️
namesys/utilities.go	94.04% <100.00%> (+3.25%)	⬆️
namesys/namesys.go	57.89% <37.50%> (-2.11%)	⬇️

... and 4 files with indirect coverage changes

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