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_new.html

@@ -49,6 +49,9 @@
   <wn-field-row label="Degeneration policy">
     <wn-input name="degeneration_policy" value="trsl:policy:linear-longstop-v0@1.0" required></wn-input>
   </wn-field-row>
+  <p style="margin-top:-0.5rem;color:#888;font-size:0.85rem;">
+    <a href="/reference/policies/linear-longstop-v0">view the linear-longstop-v0 spec</a>
+  </p>
   <wn-field-row label="Longstop date (ISO 8601)">
     <wn-input name="longstop_at" required></wn-input>
   </wn-field-row>