Preserve IPv6 scope zone in pairing addresses and peer URLs

[mkalkere]

Description

Fixes #69: pairing failed after a correct PIN when the inviter connected over IPv6 link-local.

Root cause is not TLS, as first suspected -- the pairing client pins the server key (DER compare, no hostname verification), so the handshake was fine. The failure is visible in the reporter's log: outboundIP() rendered the local address with ip.String(), dropping the IPv6 scope zone. The inviter advertised [fe80::...]:14321 with no zone, so after the PIN was accepted the joiner's Completion Exchange POST died with no route to host -- the address was unroutable without %en0.

The fix, in services/nvpair-cluster-manager: (1) outboundIP now preserves the zone via a new scopedHost helper; (2) a new peerURL helper renders peer addresses into URLs through net/url, percent-encoding the zone per RFC 6874. The second part is required, not cosmetic: Go's url.Parse rejects a raw %en0 as an invalid URL escape, so preserving the zone alone would have traded a dial error for a parse error. Applied at all three raw string-concat URL sites: pairing POST (invite.go), removal notify (mtls.go), roster reconcile (roster_http.go).

Scope

Included: the three call sites above, pairing_linklocal_test.go, version bump, docs/pairing-ipv6-link-local.mdx. Excluded: the same raw-concat pattern in nvpair-workload-manager/broadcast.go:127 and nvpair-errors/peersync.go:280 -- same latent defect, different components, left for a focused follow-up.

Validation

• go test -race ./... in services/nvpair-cluster-manager: pass, except TestReachableEndpointFirstLeadsWithTheAddressThatAnswers, which fails identically on pristine upstream (sandbox networking, unrelated to this change).
• New tests (5): zone preserved through outboundIP, RFC 6874 rendering round-trips, all three call sites produce parseable URLs for zone'd addresses.
• Toolchain: Go 1.26.8 (repo requires Go 1.25+), Linux sandbox. go vet clean, gofmt clean, node scripts/spdx-headers.mjs reports 0 missing headers on every branch.

Risk

• URL rendering now percent-encodes the zone (%25en0); required for url.Parse to accept it. No behavior change for IPv4 or loopback addresses.

Checklist

• I have read the Contributing Guidelines.
• Every commit is signed off (git commit -s), certifying the Developer Certificate of Origin.
• New or existing tests cover the change.
• Relevant documentation is updated.
• I checked the diff, changed filenames, and commit messages for credentials, private data, internal URLs, internal issue identifiers, and generated artifacts.
• I recorded the validation commands and results above.
• I bumped any affected component in services/versions.json, and described user-visible changes above so they reach the release notes.