Phase 3: Client-side routing with SX page registry + routing analyzer demo

Add client-side route matching so pure pages (no IO deps) can render
instantly without a server roundtrip. Page metadata serialized as SX
dict literals (not JSON) in <script type="text/sx-pages"> blocks.

- New router.sx spec: route pattern parsing and matching (6 pure functions)
- boot.sx: process page registry using SX parser at startup
- orchestration.sx: intercept boost links for client routing with
  try-first/fallback — client attempts local eval, falls back to server
- helpers.py: _build_pages_sx() serializes defpage metadata as SX
- Routing analyzer demo page showing per-page client/server classification
- 32 tests for Phase 2 IO detection (scan_io_refs, transitive_io_refs,
  compute_all_io_refs, component_pure?) + fallback/ref parity
- 37 tests for Phase 3 router functions + page registry serialization
- Fix bootstrap_py.py _emit_let cell variable initialization bug
- Fix missing primitive aliases (split, length, merge) in bootstrap_py.py

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

shared/sx/ref/sx_ref.py

@@ -876,6 +876,9 @@ range = PRIMITIVES["range"]
 apply = lambda f, args: f(*args)
 assoc = PRIMITIVES["assoc"]
 concat = PRIMITIVES["concat"]
+split = PRIMITIVES["split"]
+length = PRIMITIVES["len"]
+merge = PRIMITIVES["merge"]
 
 
 # =========================================================================
@@ -1237,6 +1240,40 @@ compute_all_io_refs = lambda env, io_names: for_each(lambda name: (lambda val: (
 component_pure_p = lambda name, env, io_names: empty_p(transitive_io_refs(name, env, io_names))
 
 
+# === Transpiled from router (client-side route matching) ===
+
+# split-path-segments
+split_path_segments = lambda path: (lambda trimmed: (lambda trimmed2: ([] if sx_truthy(empty_p(trimmed2)) else split(trimmed2, '/')))((slice(trimmed, 0, (length(trimmed) - 1)) if sx_truthy(((not sx_truthy(empty_p(trimmed))) if not sx_truthy((not sx_truthy(empty_p(trimmed)))) else ends_with_p(trimmed, '/'))) else trimmed)))((slice(path, 1) if sx_truthy(starts_with_p(path, '/')) else path))
+
+# make-route-segment
+make_route_segment = lambda seg: ((lambda param_name: (lambda d: _sx_begin(_sx_dict_set(d, 'type', 'param'), _sx_dict_set(d, 'value', param_name), d))({}))(slice(seg, 1, (length(seg) - 1))) if sx_truthy((starts_with_p(seg, '<') if not sx_truthy(starts_with_p(seg, '<')) else ends_with_p(seg, '>'))) else (lambda d: _sx_begin(_sx_dict_set(d, 'type', 'literal'), _sx_dict_set(d, 'value', seg), d))({}))
+
+# parse-route-pattern
+parse_route_pattern = lambda pattern: (lambda segments: map(make_route_segment, segments))(split_path_segments(pattern))
+
+# match-route-segments
+def match_route_segments(path_segs, parsed_segs):
+    _cells = {}
+    return (NIL if sx_truthy((not sx_truthy((length(path_segs) == length(parsed_segs))))) else (lambda params: _sx_begin(_sx_cell_set(_cells, 'matched', True), _sx_begin(for_each_indexed(lambda i, parsed_seg: ((lambda path_seg: (lambda seg_type: ((_sx_cell_set(_cells, 'matched', False) if sx_truthy((not sx_truthy((path_seg == get(parsed_seg, 'value'))))) else NIL) if sx_truthy((seg_type == 'literal')) else (_sx_dict_set(params, get(parsed_seg, 'value'), path_seg) if sx_truthy((seg_type == 'param')) else _sx_cell_set(_cells, 'matched', False))))(get(parsed_seg, 'type')))(nth(path_segs, i)) if sx_truthy(_cells['matched']) else NIL), parsed_segs), (params if sx_truthy(_cells['matched']) else NIL))))({}))
+
+# match-route
+match_route = lambda path, pattern: (lambda path_segs: (lambda parsed_segs: match_route_segments(path_segs, parsed_segs))(parse_route_pattern(pattern)))(split_path_segments(path))
+
+# find-matching-route
+def find_matching_route(path, routes):
+    _cells = {}
+    path_segs = split_path_segments(path)
+    _cells['result'] = NIL
+    for route in routes:
+        if sx_truthy(is_nil(_cells['result'])):
+            params = match_route_segments(path_segs, get(route, 'parsed'))
+            if sx_truthy((not sx_truthy(is_nil(params)))):
+                matched = merge(route, {})
+                matched['params'] = params
+                _cells['result'] = matched
+    return _cells['result']
+
+
 # =========================================================================
 # Fixups -- wire up render adapter dispatch
 # =========================================================================