Implement flexible entity relation system (Phases A–E)

Declarative relation registry via defrelation s-expressions with
cardinality enforcement (one-to-one, one-to-many, many-to-many),
registry-aware relate/unrelate/can-relate API endpoints, generic
container-nav fragment, and relation-driven UI components.

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

shared/sexp/parser.py

@@ -46,7 +46,7 @@ class Tokenizer:
     COMMENT = re.compile(r";[^\n]*")
     STRING = re.compile(r'"(?:[^"\\]|\\.)*"')
     NUMBER = re.compile(r"-?(?:\d+\.?\d*|\.\d+)(?:[eE][+-]?\d+)?")
-    KEYWORD = re.compile(r":[a-zA-Z_][a-zA-Z0-9_-]*")
+    KEYWORD = re.compile(r":[a-zA-Z_][a-zA-Z0-9_>-]*")
     # Symbols may start with alpha, _, or common operator chars, plus ~ for components,
     # <> for the fragment symbol, and & for &key/&rest.
     SYMBOL = re.compile(r"[a-zA-Z_~*+\-><=/!?&][a-zA-Z0-9_~*+\-><=/!?.:&]*")