Promote literal-discriminated unions to a first-class kind

[guybedford]

A type alias type Foo = A | B | … whose branches share a required, literal-typed property now lands in a new TypeKind::DiscriminatedUnion rather than merging into a flat InterfaceDecl. The discriminator can be a string, number, or boolean literal — the three forms TS narrows on.

The motivation is per-branch required-vs-optional accuracy. With the previous merged-shape model, EmailAttachment.contentId (required in the inline branch, ?: undefined in attachment) collapsed to merged-optional, so new_inline(...) didn't take it as a parameter even though the source contract requires it:

// before
EmailAttachment::new_inline(filename: &str, type_: &str, content: &str)
// after
EmailAttachment::new_inline(content_id: &str, filename: &str, type_: &str, content: &str)
EmailAttachment::new_attachment(filename: &str, type_: &str, content: &str)

The codegen keeps a per-branch view:

• Extern block stays merged — one pub type Foo plus a getter/setter set per property across all branches. No change for downstream consumers reaching for the bare type.
• impl Foo emits one new_<discriminator> / builder_<discriminator> cohort per branch, driven by that branch's required set. Each branch independently goes through the literal-collapse / value-union expansion / _with_<type> suffixing rules.
• Wrapper builder is still merged — EmailAttachmentBuilder::content_id(self, val) is always available so callers going through builder_attachment(...).content_id(x).build() aren't blocked. The branch invariant is enforced at the new_<discriminator> boundary; once past that, the wrapper exposes the runtime-permissive view of the type.
• builder_<branch> suppression: when a branch's required set already covers every merged-optional field, the builder_<branch> companion would have nothing to chain — so new_<branch> is emitted with an inlined body and no builder. For EmailAttachment, inline covers the only merged-optional (contentId) so it gets an inlined body, while attachment leaves contentId for the wrapper and keeps its builder_attachment companion.

Implementation:

• parse/literal_union.rs::detect_discriminators — a property qualifies when it's required and string/number/boolean-literal-typed in every branch.
• parse/first_pass/populate.rs::try_synthesize_alias_decl — returns TypeKind (was InterfaceDecl); routes to DiscriminatedUnion when discriminators are detected, plain Interface otherwise.
• codegen/classes.rs — generate_dictionary_factory is now a thin wrapper around generate_dictionary_factory_with_passes, which takes an optional list of required-field passes. Plain interfaces pass nothing and behave as before; the new module passes one entry per branch. A shared emitted_names set deduplicates combos that converge across passes, and the per-pass pass_covers_optionals check drives the new builder_<branch> suppression.
• codegen/discriminated_unions.rs — new module wiring DiscriminatedUnionDecl through the shared infra.

Snapshot diffs are limited to discriminated unions and dictionaries that previously landed alphabetical (now source-order, per #21). Plain interfaces are unchanged. CONVENTIONS.md gains a ## Discriminated unions section spelling out the detection rule, the per-branch factory shape, the wrapper-stays-merged invariant, and the builder_<branch> suppression rule.