Remove dead dialect methods; unblock explicit-creds tag-metadata path

Two cleanups from the post-EnsureTagAsync punch list — both isolated, no
protocol discovery required.

#89 dead code in Historian2020ProtocolDialect:
  - BrowseTagNamesAsync and GetTagMetadataAsync on the dialect both threw
    ProtocolEvidenceMissingException, but HistorianClient routes those calls
    directly to HistorianWcfTagClient — the dialect overrides were never
    reached. Removed both methods. ReadBlocksAsync stays (it's a deliberate
    guardrailed entry on the public surface).

#90 explicit-creds tag-metadata path:
  - HistorianWcfTagClient.WcfRetrievalSession.ValidateSupportedAuth threw
    ProtocolEvidenceMissingException whenever IntegratedSecurity=false AND
    UserName/Password were supplied. But the surrounding code already wires
    those creds through ApplyWindowsCredential ->
    factory.Credentials.Windows.ClientCredential — the validator was just
    being conservative about an untested combination.
  - Inverted the check: now only rejects the no-auth-at-all combination
    (IntegratedSecurity=false + no UserName + no Password). The other three
    valid auth shapes pass through to WCF.

Tests: 161 -> 163 (+2). New unit test verifies the no-auth case still
throws; new gated live integration test
GetTagMetadataAsync_ExplicitCredentials_AgainstLocalHistorian exercises the
explicit-creds path when HISTORIAN_USER+HISTORIAN_PASSWORD are set, skips
cleanly otherwise.

CLAUDE.md updated: removed the two now-resolved entries from "Remaining
gaps"; explicit-creds line refined to note the live-verification env-var
requirement.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -94,8 +94,7 @@ End-to-end chain working from a pure managed .NET 10 client: `Hist.GetV → Hist
 Smaller, isolated items — none block the production read surface:
 
 - Remote TCP transports (`RemoteTcpIntegrated`, `RemoteTcpCertificate`) untested against an actual remote Historian (tests skip without `HISTORIAN_REMOTE_TCP_HOST`).
-- Explicit username/password tag-metadata path (`HistorianWcfTagClient` line ~357) throws — only integrated security wired for that op.
-- `Historian2020ProtocolDialect.GetTagInfoByName/GetTagInfos` throws — currently dead code; `GetTagMetadataAsync` works through the WCF tag client instead.
+- Explicit username/password tag-metadata path is wired (validator only blocks no-auth-at-all), but live-verification requires `HISTORIAN_USER`+`HISTORIAN_PASSWORD` set; gated test `GetTagMetadataAsync_ExplicitCredentials_AgainstLocalHistorian` skips otherwise.
 - Per-row trailing ~24 bytes of `GetNextQueryResultBuffer` are not decoded (likely per-sample value/source/state metadata).
 - `EnsureTagAsync` distinct `MinRaw`/`MaxRaw` persistence requires `ApplyScaling=true` + a follow-up `UpdateTags` call — not yet wired (no API user has asked).