Add interactive Matrix SAS verification

[TroyHernandez]

Summary

Add standard interactive Matrix SAS verification from an R console, plus
event-loop integration and pinned directional user verification.

• Human comparison of emoji or decimal values, modern SAS key agreement and
  MACs, cancellation/timeouts, stable outboxes, fixed key snapshots, and
  read-back-confirmed trust uploads.
• Cross-user verification requires the peer master proof as well as its
  device proof. Same-account verification authenticates/signs the new device
  under the existing locally pinned identity.
• Preserve verification envelopes and relations through encryption; save
  ratchet state and cursor before transport or trust uploads. Console retries
  reload persisted config so a stale R object cannot rewind sync.
• Record the reproducible console and peer-identity recovery procedures in
  README, vignette, and the package-owned Matrix messaging skill. No private
  client database extraction is required. Operational incident details remain
  outside the public repository.

Dependencies and commit boundaries

SAS requires optional mx.crypto >= 0.2.1.2. Existing E2EE floors remain
mx.api >= 0.3.0.2 and mx.crypto >= 0.2.1.1; neither is raised by this PR.
The 0.2.0.10 bump is a separate DESCRIPTION/NEWS commit. Please preserve that
boundary when merging.

mx.crypto PR #8 is merged. CI pins its main commit
7feb052ab40e7ac5620b4d405635a2bc4905984c, reads the older dependency floors
from DESCRIPTION, builds vignettes, fails on check warnings, and explicitly
requires nonzero passing results from each SAS/identity test file. No drat
publication is needed for this change.

Validation

• Fresh isolated build: 683/683 source assertions pass.
• Local R CMD check --as-cran --no-manual: 0 errors, 0 warnings, 0 notes.
• Independent Matrix Dart SDK 10.2.0 interoperation passes in both roles.
  Matching displays with an omitted master proof are correctly rejected.
• CI-script controls exercise all 278 SAS/identity assertions and reject
  mutable dependency pins and warning check results.
• Windows development check will run after merge.

Package reference pages on the separate website have not been updated or
deployed as part of this change.