Implement builder (ADR-009) and validate_bytes (ADR-010) for v0.1.0

POC: /workspace/alktype-builder-poc/ (11/11 tests pass, findings in
docs/research/alktype-builder-poc/findings.md). The POC code lives outside
the repo per the internal dev convention.

Implementation:
- src/builder.rs: Schema, Definitions, Discriminator types. Constructors
  for all 19 AlkType kinds + standard JSON Schema types (object/array/
  string/integer/number/boolean/null/any). Setters for ADR-003 annotations
  (endian/align/encoding/max_length), composite builders (field/required/
  items/mapping), and standard JSON Schema constraints (minimum/maximum/
  minLength/minItems/maxItems/format/title/description). 16 unit tests.
- src/materialize.rs: materialize_packed and materialize_aligned functions
  that walk a schema + buffer to produce a serde_json::Value tree. Recurses
  into Struct, Array, Union (byte-offset discriminator). Record is stubbed
  (deferred for the POC scope).
- src/engine.rs: AlkTypeEngine::validate_bytes(&[u8]) added (ADR-010).
  Dispatches on layout mode, materializes Value, then validates against
  the existing jsonschema validator. 7 unit tests.
- src/lib.rs: pub mod builder, pub mod materialize; re-exports Schema,
  Definitions, Discriminator.

Verification:
- cargo test: 346 -> 369 tests pass (23 new: 16 builder, 7 validate_bytes)
- cargo clippy --all-targets -- -D warnings: clean
- POC (11 tests): builder round-trip + validate_bytes (packed + aligned) +
  validate_json for call payloads; all pass

Findings:
- Top-level schema must be AlkType:Struct (existing constraint); unions are
  field types within a struct. Builder spec Example 3 needs a doc fix.
- Builder field order preserved (preserve_order feature, load-bearing for
  packed mode).
- validate_bytes correctly distinguishes Access (read phase) from
  Validation (validate phase) errors, with field paths.
- validate_json path unchanged for call payloads.

Open questions surfaced (OQ-005, OQ-006, OQ-007) tracked in findings.md;
to resolve before the SFTP Packet POC round.

src/lib.rs

@@ -18,21 +18,29 @@
 //!   discriminator dispatch.
 //! - **Validation** ([`validation`]): Custom keyword validators for all
 //!   19 `AlkType:*` kinds, delegated to the `jsonschema` crate.
+//! - **Builder** ([`builder`]): Fluent Rust API for constructing alktype
+//!   JSON Schemas at runtime, producing `serde_json::Value` (ADR-009).
+//! - **Materialize** ([`materialize`]): Materialize a `serde_json::Value`
+//!   tree from a binary buffer by walking the schema. Used by
+//!   `AlkTypeEngine::validate_bytes` (ADR-010).
 //! - **Engine** ([`engine`]): `AlkTypeEngine` — the compiled form of a
 //!   schema, combining layout and validation.
 
 #[macro_use]
 mod macros;
+pub mod builder;
 pub mod data_access;
 pub mod engine;
 pub mod error;
 pub mod layout_builder;
+pub mod materialize;
 pub mod offset_map;
 pub mod schema;
 pub mod sequential_reader;
 pub mod tunion;
 pub mod validation;
 
+pub use builder::{Definitions, Discriminator, Schema};
 pub use engine::{LayoutMode, AlkTypeEngine};
 pub use error::AlkTypeError;
 pub use layout_builder::{FieldPosition, LayoutBuilder, PackedLayout};