feat(reflect): generic value tree + Reflect.serialize/apply + JSON bridge (#44)

A self-describing Value node (null/int/fixed/bool/str/list/object) with
constructors, builders (Value.add/put) and accessors (get/at/count/kind/
as_int/as_str/…). Reflect.serialize(entity) walks an entity's whole component
set into a value tree — one member per component, each a sub-object of its
fields — and Reflect.apply(entity, value) writes one back; a fixed field
becomes a fixed node, everything else an int node, so the round-trip is
bit-exact, with the model id under "@kind". Json.encode/parse bridge the tree
to and from compact, stable, diffable text, with fixed written as an exact
terminating decimal that parses back bit-for-bit (verified across the raw
Q16.16 range). Together: a one-call, bit-exact save/load for entities.

Written in Ludic and spliced on demand (runtime/native/value.ludic +
reflect_io.ludic, like Query/Light), so a program that doesn't touch
Value.*/Json.*/Reflect.serialize compiles byte-identically and the C-free
bootstrap fixpoint holds (verified). The general tagged-union/any language type
stays tracked in #1; this ships the concrete value tree the serializer needs.

Adds 21 namespace-method docs pages + Value/Json sections,
examples/library/serialize.ludic, and a regression case. Whole CI set green:
x test 72/72, x test-tools 30/30, check-impl/vocabulary/docs.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/language/json/json-encode.md

@@ -0,0 +1,14 @@
+---
+id: json-encode
+name: Json.encode
+category: json
+kind: namespace-method
+tokens: Json.encode
+sig: Json.encode(value) -> string
+tip: Serialize a value tree to compact JSON text.
+order: 1
+ns: Json
+member: encode
+---
+
+Walks a value tree and returns compact JSON text. <code>int</code> nodes become integers, <code>fixed</code> nodes an exact decimal, <code>bool</code> <code>true</code>/<code>false</code>, <code>str</code> a quoted (escaped) string, and lists/objects the usual <code>[…]</code>/<code>{…}</code> — object members keep insertion order, so the output is stable and diffable. The inverse is <a href="json-parse"><code>Json.parse</code></a>; together with <a href="reflect-serialize"><code>Reflect.serialize</code></a> this is a one-call save.