Rust AST → TypeScript emitter for ontogen's long-tail type bindings.

This crate is the build-time replacement for the OF-014 spike's specta side-car. Given a set of root types, a pool of candidate type definitions (syn::Item keyed by canonical [TypePath]), and an [EmitConfig], it produces TypeScript source covering the supported subset documented in OF-015:

• Named structs over primitive / container / smart-pointer / reference field types
• C-style enums (and tagged enums where the tag is implicit from variant idents)
• Vec<T>, Option<T>, HashMap<K, V>, BTreeMap<K, V>
• Primitives: bool, all integer types, f32/f64, String, &str
• Smart-pointer wrappers (Box, Rc, Arc, Cow, Pin) peeled silently
• External types via [EmitConfig::external_types]
• #[serde(flatten)] on a struct field (or an enum struct-variant field), emitted as a TS intersection: StepMeta & { program: string }
• The full rename family, each on the axis serde gives it: rename_all (a container's members), rename_all_fields (an enum's struct-variant fields), rename_all on a variant (that variant's fields), rename

See docs/tasks/OF-015-productionize-typescript-generation.md for the full design pass.

PR series state (PR 4 of 8)

PR 1-3 landed the crate scaffold, per-type emission, serde rename family, type-pool walker, use-resolution, external-types table, and topological ordering. PR 4 wires them together: the top-level [emit] function now composes collection → name resolution → collision detection → topological ordering → per-type emission → error aggregation, all in one pass. The #[ts_opaque(target = "...")] and #[ts_name = "..."] proc-macro attrs (shipped in ontogen-macros) are read here to short-circuit emission for opaque types and override TS names. External-types lookup is wired into emit_type's fall-through so types like chrono::DateTime resolve to string per the shipped defaults.

PR 5 wires ontogen itself to call [emit] instead of the side-car emitter.