ci: provision Pi runtime 0.84.1 in the test step (Invariant R)

invariant_r_unittest.py (landing with the lease-remediation stack, PR
#1109) hard-requires an installed `pi` binary pinned to the measured
version: it boots Pi's real tool registry and proves the broker's
read-only carve-out resolves to real, unshadowed builtins. Absent
runtime fails loud by design — so CI must provide it.

Install @earendil-works/[email protected].1 (the canonical Pi;
@mariozechner/* is embedded-legacy) at step level in the test step.
Step-level rather than baked into Dockerfile.ci because ci-image
publishes are currently blocked on registry UNAUTHORIZED; baking it in
is the follow-up once registry auth is fixed, at which point this line
degrades to a fast no-op guard like the openssl line above it.

Co-Authored-By: Claude Fable 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01Dtdjx4Gxude9fwyLezCrhh

.woodpecker/ci.yml

@@ -109,6 +109,16 @@ steps:
       # `apk add` guarantees openssl is present on PR pipelines too (and is a
       # fast no-op once the rebuilt image already ships it).
       - apk add --no-cache openssl
+      # Pi runtime (Invariant R): invariant_r_unittest.py hard-requires an
+      # installed `pi` binary at exactly this measured version — the test
+      # boots Pi's real tool registry to prove the read-only carve-out
+      # resolves to real, unshadowed builtins, and fails loud (by design)
+      # when the runtime is absent or drifts. The canonical Pi is
+      # @earendil-works/[email protected] exactly (@mariozechner/* is
+      # embedded-legacy). Step-level install because ci-base image publishes
+      # are currently blocked on registry auth; fold into Dockerfile.ci once
+      # that is fixed, keeping this as a fast no-op guard.
+      - npm install -g @earendil-works/[email protected]
       # postgresql-client (pg_isready) is baked into ci-base.
       # Wait up to 60s for CI postgres to be ready; fail fast if it never comes up.
       - |