Add a `--tpt-proto=tipc` CI matrix leg

Plan 01 §7.3's last item. The module ships w/ the standard
ubuntu kernel package but is NOT loaded by default, so the leg
gets a gated `sudo modprobe tipc` step plus a verify that
asserts `TIPCAddress.is_available()` before the suite runs —
i.e. a missing module fails w/ an actionable line instead of a
few hundred connect timeouts.

Deats,
- `tipc` added to the `tpt_proto` matrix axis, and excluded on
  `macos-latest` bc `AF_TIPC` is a linux-kernel proto that
  doesn't exist on darwin at all.
- `continue-on-error` is scoped to just this leg via
  `${{ matrix.tpt_proto == 'tipc' }}` — GH's runners have never
  been asked to `modprobe` for us, so it lands NON-blocking
  until it's had a few green runs. Drop the gate then.
- if the runners do refuse, the documented fallback is a
  container job w/ `--cap-add NET_ADMIN`.

Cross-node (bearer) TIPC still can't be CI'd; that stays the
manual smoke test in `examples/multihost/tipc_cluster/README.md`.

Partially addresses #420.

(this patch was generated in some part by `claude-code` using `claude-opus-5` (`anthropic`))

.github/workflows/ci.yml

@@ -92,6 +92,12 @@ jobs:
     timeout-minutes: 16
     runs-on: ${{ matrix.os }}
 
+    # XXX the `tipc` leg stays NON-blocking until the `modprobe`
+    # step below is proven stable on GH's runners; flip this off
+    # once it's had a few green runs. See plan 01 §7.3 in
+    # `ai/tpt-backends/01_tipc_backend.md`.
+    continue-on-error: ${{ matrix.tpt_proto == 'tipc' }}
+
     strategy:
       fail-fast: false
       matrix:
@@ -117,6 +123,7 @@ jobs:
         tpt_proto: [
           'tcp',
           'uds',
+          'tipc',
         ]
         # https://github.com/orgs/community/discussions/26253#discussioncomment-3250989
         exclude:
@@ -124,6 +131,11 @@ jobs:
           - os: macos-latest
             tpt_proto: 'uds'
 
+          # XXX `AF_TIPC` is a linux-kernel protocol; the address
+          # family doesn't exist on darwin at all.
+          - os: macos-latest
+            tpt_proto: 'tipc'
+
     steps:
       - uses: actions/checkout@v4
 
@@ -150,6 +162,20 @@ jobs:
       - name: List deps tree
         run: uv tree
 
+      # XXX `AF_TIPC` needs its kernel module loaded! It ships
+      # with the standard ubuntu kernel package but is NOT loaded
+      # by default, so a bare `--tpt-proto=tipc` run would
+      # otherwise fail at the first `.bind()`.
+      #
+      # If GH's runners ever refuse the `modprobe`, the fallback
+      # is a container job w/ `--cap-add NET_ADMIN`.
+      - name: 'Load the `tipc` kernel module'
+        if: matrix.tpt_proto == 'tipc'
+        run: |
+          sudo modprobe tipc
+          tipc node get address
+          uv run python -c "from tractor.ipc._tipc import TIPCAddress; ok, why = TIPCAddress.is_available(); assert ok, why; print('AF_TIPC available')"
+
       - name: Run tests
         run: >
           uv run