iter 0001.tidy: registry.toml, cache-before-fetch, escape registry slugs

Resolves the actionable drift the cycle-close architect review found:
- Ship registry.toml (the spec's acceptance artifact) for aura + data-server,
  so `docsite serve`/`check` run out of the box — the headline live-render was
  not runnable as shipped without it.
- Cache before fetch: the page handler now resolves the branch SHA, serves
  from cache on a hit, and only fetches the source on a miss (the spec
  data-flow). The pre-tidy handler always fetched, paying a wasted Gitea read
  on every cache hit. A new e2e test (a counting fetcher) pins that a second
  request serves from cache without re-fetching.
- Escape the registry slug/page in nav_html's href and the index links —
  operator-trusted data, but consistent with the documented no-injection
  invariant and the rest of the shell (which already escaped).

cargo test green (41: 32 lib + 8 e2e + 1 gated live smoke); clippy clean.

refs #1

registry.toml

@@ -0,0 +1,32 @@
+# DocSite registry — the publish whitelist and per-project Gitea coordinates.
+# Only pages listed here are reachable; everything else (docs/specs, docs/plans,
+# postmortems, internal files) is structurally unservable. DocSite reads these
+# sources read-only from Gitea and never writes to them.
+
+bind  = "0.0.0.0:8080"
+gitea = "http://192.168.178.103:3000"
+# The read-only Gitea token is resolved from $DOCSITE_GITEA_TOKEN, never stored here.
+
+[[project]]
+slug   = "aura"
+repo   = "Brummel/aura"
+branch = "main"
+docs   = "docs"
+
+  [[project.section]]
+  title = "Concepts"
+  pages = ["glossary.md"]
+
+  [[project.section]]
+  title = "Design & Architecture"
+  pages = ["design/INDEX.md"]
+
+[[project]]
+slug   = "data-server"
+repo   = "Brummel/data-server"
+branch = "main"
+docs   = "."
+
+  [[project.section]]
+  title = "Reference"
+  pages = ["README.md"]