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/_section.md

@@ -0,0 +1,7 @@
+---
+id: json
+title: Json
+order: 36
+---
+
+The text bridge over the <a href="value"><code>Value</code></a> tree: <a href="json-encode"><code>Json.encode</code></a> turns a value tree into compact, stable JSON text and <a href="json-parse"><code>Json.parse</code></a> reads it back. Together with <a href="reflect-serialize"><code>Reflect.serialize</code></a>/<a href="reflect-apply"><code>Reflect.apply</code></a> this is a one-call, bit-exact save/load for entities, and a diffable on-disk format for tooling. Determinism holds: the same tree always encodes to the same bytes.