Add deps and engine test specs, bootstrap engine to Python

New test specs (test-deps.sx: 33 tests, test-engine.sx: 37 tests) covering
component dependency analysis and engine pure functions. All 6 spec modules
now have formal SX tests: eval (81), parser (39), router (18), render (23),
deps (33), engine (37) = 231 total.

- Add engine as spec module in bootstrap_py.py (alongside deps)
- Add primitive aliases (trim, replace, parse_int, upper) for engine functions
- Fix parse-int to match JS parseInt semantics (strip trailing non-digits)
- Regenerate sx_ref.py with --spec-modules deps,engine
- Update all three test runners (run.js, run.py, sx-test-runner.js)
- Add Dependencies and Engine nav items and testing page entries
- Wire deps-source/engine-source through testing overview UI

Node.js: 231/231 pass. Python: 226/231 (5 pre-existing parser/router gaps).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

sx/sx/nav-data.sx

@@ -113,6 +113,8 @@
   (dict :label "Parser" :href "/testing/parser")
   (dict :label "Router" :href "/testing/router")
   (dict :label "Renderer" :href "/testing/render")
+  (dict :label "Dependencies" :href "/testing/deps")
+  (dict :label "Engine" :href "/testing/engine")
   (dict :label "Runners" :href "/testing/runners")))
 
 (define isomorphism-nav-items (list