types/node, policy/v2: drop taildrive caps from baseline emission

Taildrive (drive:share and drive:access) is policy-driven per
Tailscale's documented behaviour
(https://tailscale.com/docs/features/taildrive). The previous
always-on baseline emission diverged from SaaS for every node not
targeted by a drive nodeAttr -- a real semantic divergence that the
compat suite caught once the test moved to comparing TailNode output
against the captured netmaps.

types.Node.TailNode no longer stamps the drive pair. Operators
wanting taildrive add a nodeAttrs entry:

  "nodeAttrs": [
    { "target": ["*"], "attr": ["drive:share", "drive:access"] }
  ]

unmodelledTailnetStateCaps shrinks accordingly. The baseline-divergence
group is gone; every entry left in the list is genuinely unmodelled
(user-role caps, unimplemented features, tailnet metadata, internal
tuning).

servertest's TestNodeAttrsBaselineCapsAlwaysOn expects the smaller
baseline (admin + ssh + file-sharing). Integration TestGrantCapDrive
grants the drive caps explicitly via NodeAttrs to exercise the
policy-driven emission path.