docs(typedef): sync specs with the implemented API surface

The code introduced concrete public types and a unified API during
implementation that the specs described only conceptually. Sync the
specs to match the code:

- schema-layer.md: document the TypeDefKind enum and its inherent methods;
  add a Schema-Layer Public API section (get_typedef_kind vs
  get_typedef_kind_loose, annotation parsers, Endian/VariableEncoding/
  DiscriminatorKind, normalize_refs/resolve_ref/resolve_ref_or_inline);
  fix the TRecord layout (values are encoded by their declared kind, not
  universally value_len-prefixed); fix the alignment default list.
- layout-engine.md: document the LayoutMode enum and the ByteRange/
  FieldPosition/PackedLayout/OffsetMap public types with their actual
  signatures; update the LayoutBuilder/SequentialReader/OffsetMap
  component descriptions with the real new/build/compute signatures.
- data-access.md: document the FieldValue enum; add a Higher-level
  read/write section (TypedefEngine::read_field/write_field,
  SequentialReader::read_next/read_field); fix primitive signatures to
  include field_path and Endian; replace the wrong
  read_union_discriminator pseudo-code with the actual tunion module API
  (read_byte_discriminator/read_field_discriminator/resolve_variant/
  discriminator_size) and the UnionDispatch struct.
- validation.md: fix the TypedefEngine struct (add endian/schema fields,
  mark Layout as private); add the real compile signature (&mut Value,
  LayoutMode) and mode-appropriate accessors; fix engine.validate(buffer)
  -> engine.validate_json(&Value)/is_valid_json (the validator operates
  on serde_json::Value, not byte buffers — matches ADR-098).
- overview.md: remove the stale ~1,900 lines / 26 tests line count.
- ADR-097 §3a: correct the TRecord layout (no separate value_len prefix;
  the value is encoded by its declared TypeDef:* kind).

docs/architecture/decisions/097-schema-annotations.md

@@ -144,8 +144,15 @@ the `"values"` property in the schema:
 - `"values"` is a schema object declaring the `TypeDef:*` kind of all
   values in the record. All values share the same type.
 - The binary layout is a count-prefixed sequence of `(key, value)` pairs:
-  `[count: u32][key_len: u32][key_bytes][value_len: u32][value_bytes]...`.
-- The count prefix respects the schema's endianness.
+  `[count: u32][key_len: u32][key_bytes][value]...` repeated `count`
+  times. Each key is a length-prefixed UTF-8 string. Each value is
+  encoded according to its declared `TypeDef:*` kind — a `Record<Uint32>`
+  value is 4 raw bytes; a `Record<String>` value is itself a
+  length-prefixed string; a `Record<Struct>` value is the struct's
+  fields laid out inline. There is **no separate `value_len` prefix** —
+  the value's size is determined by its kind (fixed-size kinds have a
+  known size; variable-length kinds carry their own length prefix).
+- The count and key-length prefixes respect the schema's endianness.
 - In aligned static mode with `maxLength`, the entire record is reserved
   at `maxLength` bytes (zero-padded).