fix(lint): clear develop golangci-lint debt

[0pcom]

Clears the remaining golangci-lint findings on develop so CI lint goes green: errcheck, gosec (G404), misspell, staticcheck (SA1019/SA4006/SA4023), unparam, and ineffassign.

Notable fixes:

• errcheck: check the errors from pk.Set/srv.Set in a disc test (check-blank enabled).
• gosec G404: justified //nolint:gosec on non-crypto shuffles used for peer/service/collection ordering (load distribution, not security-sensitive).
• misspell: marshalled -> marshaled in CXO-publisher comments.
• staticcheck SA1019: http.Transport.Dial -> DialContext via proxy.ContextDialer; drop the deprecated no-op svcmode.Config.DisableDHT field.
• staticcheck SA4023: remove always-true err != nil comparisons on functions that only ever return a non-nil error (blocking Serve/accept loops, the ensureLoginChain stub).
• staticcheck SA4006: check the previously-discarded entry.String() error; drop a dead j--.
• unparam: drop the always-nil error return from encodeServicesBatch/encodeClientsBatch and update all callers; justified //nolint:unparam on a test closure whose signature is required by registry.IterateFunc.

Verified: golangci-lint run ./... reports 0 issues, go build . succeeds, and go test passes for every package whose non-test code changed.

Developed with AI assistance (Claude).