Implement WP-0015-T03: Control Plane reference-rendering routes

New service/reference_docs.py renders specs/policies/*.md and
specs/profiles/*.md read-only at request time via a small markdown
library (added markdown + PyYAML to the service extras) -- not a
static-build pipeline, matching the WP-0012-T03 decision to skip
state-hub's heavier Observable Framework pattern.

One parameterized route, GET /reference/{kind}/{slug}, covers both
addendum URL shapes. Discovered phase_detail.html's Status table never
displayed the degeneration_policy id at all -- added that row (with
the reference link) rather than wiring a link with nothing to attach
it to. phase_new.html gets a plain link next to the field.

Deliberately did not wire extension-id links into the UI in this task
-- extension ids don't appear anywhere in the Control Plane today
(that's WP-0014's gap, not this one's to expand).

6 new Docker-gated tests. Full suite: 94 passing offline, 164 passing
with Docker (up from 158).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/target_revenue/service/control_plane_templates/phase_detail.html

@@ -24,6 +24,13 @@
   <tr><td>Outstanding Target</td><td>{{ metrics.facts.outstanding_target }}</td></tr>
   <tr><td>Target satisfaction</td><td>{{ metrics.calculations.target_satisfaction_percentage }}%</td></tr>
   <tr><td>Converted</td><td>{{ metrics.facts.is_converted }}</td></tr>
+  <tr>
+    <td>Degeneration policy</td>
+    <td>
+      {{ manifest.phase.degeneration_policy }}
+      &middot; <a href="/reference/policies/{{ policy_slug(manifest.phase.degeneration_policy) }}">view spec</a>
+    </td>
+  </tr>
 </table>
 
 <h3>Ledger ({{ ledger | length }} entries)</h3>