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.

hscontrol/policy/v2/tailnet_state_caps.go

@@ -1,22 +1,16 @@
 package v2
 
 // This file enumerates [tailcfg.NodeCapability] values that the
-// compat test in tailscale_nodeattrs_compat_test.go strips from BOTH
-// sides before [cmp.Diff]. The test builds the self-view CapMap via
-// [types.NodeView.TailNode] -- the same call the mapper makes -- so
-// every cap NOT in this list is compared in full as it lands on the
-// wire.
+// Tailscale-hosted control plane emits where headscale has no
+// equivalent concept yet. The compat test in
+// tailscale_nodeattrs_compat_test.go builds the self-view CapMap via
+// [types.NodeView.TailNode] -- the same call the mapper makes -- and
+// strips these from BOTH sides before [cmp.Diff]; every other cap is
+// compared in full as it lands on the wire.
 //
-// Entries fall into two groups:
-//  1. Caps SaaS emits that headscale has no concept of (admin / owner
-//     user roles, tailnet lock, services host, app connectors,
-//     tailnet-state metadata).
-//  2. Caps headscale emits unconditionally where SaaS gates emission
-//     on a tailnet-config knob headscale does not surface (the
-//     taildrive pair). The feature works; the gating differs.
-//
-// Each entry documents its purpose, the reason for divergence, and a
-// tracking issue where one exists.
+// Each entry documents its purpose (cross-referenced to Tailscale
+// source), why headscale does not emit it, and a tracking issue where
+// one exists.
 
 import (
 	"slices"
@@ -74,9 +68,6 @@ func PeerCapMap(peer types.NodeView, peerSelfCaps tailcfg.NodeCapMap) tailcfg.No
 //     anonymized capture.
 //  4. Caps that are internal magicsock or embedded-SSH tuning with no
 //     headscale-side equivalent.
-//  5. Baseline-divergence caps -- features headscale supports but
-//     emits unconditionally where SaaS gates on a tailnet-config
-//     toggle headscale does not surface yet.
 var unmodelledTailnetStateCaps = []tailcfg.NodeCapability{
 	// --- 1. User-role gated ---
 
@@ -164,20 +155,6 @@ var unmodelledTailnetStateCaps = []tailcfg.NodeCapability{
 	// forwarding in the embedded SSH server. Internal; default chosen
 	// by the server.
 	tailcfg.NodeAttrSSHEnvironmentVariables,
-
-	// --- 5. Baseline-divergence — feature supported, gating differs ---
-
-	// [tailcfg.NodeAttrsTaildriveShare] and
-	// [tailcfg.NodeAttrsTaildriveAccess]: the hosted control plane
-	// emits these only when policy or a tailnet-config toggle grants
-	// them. [types.Node.TailNode] emits both unconditionally so
-	// taildrive works out of the box on self-hosted tailnets. The
-	// feature is supported on both sides; only the emission gating
-	// differs. Strip until headscale grows an equivalent operator
-	// toggle (analogous to cfg.Taildrop.Enabled gating
-	// CapabilityFileSharing).
-	tailcfg.NodeAttrsTaildriveShare,
-	tailcfg.NodeAttrsTaildriveAccess,
 }
 
 // strippedCapPrefixes lists URL/string prefixes for parameterized or