Generate Term encoders for Rust types read from source

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Generates Rustler Term encoders for Rust types read from source. This covers types owned by another crate that don't implement serde, such as vize_atelier_vapor's IR, which vize_ex currently encodes by hand.

RustQ.Rustler.Term.encoders_from_source/3

Starting from root types, it follows field and variant payload types through a RustQ.Syn index and builds one free function per reachable struct or enum:

pub(crate) fn encode_set_prop_ir_node<'a>(env: rustler::Env<'a>, value: &SetPropIRNode<'_>) -> rustler::Term<'a>

• Structs encode as atom-keyed maps, and newtype structs as their field.
• Unit variants encode as atoms, tuple variants as their payload, and named variants as maps. With tag:, data-carrying variants also carry the variant atom.
• Option encodes None as nil, sequences and sets as lists, and maps as maps.
• Policy is explicit: wrapper roles (for example allocator-aware Vec/Box), external: helpers, and per-type except:, key and variant renames, and with: helpers.
• The traversal is closed. A reachable type that isn't indexed, a wrapper, a scalar, or external fails generation with a list of the unmapped types. The same applies to names defined in several sources and to generic types. An upstream change therefore shows up at generation time.
• A tag: that would replace a payload field with the same key fails generation instead of silently overwriting it.
• transparent: true encodes a struct with exactly one field as that field.
• encoder_atoms_from_source/3 returns the atom declarations from the same source.

The feature is documented in a new "Encoders for types from Rust source" section of the Rustler generation guide.

Supporting changes

• Syn metadata: RustQ.Syn.Enum.variant_shapes gives one RustQ.Syn.Variant per variant, with its kind, fields, and docs. lifetimes and type_parameters are added on Syn.Struct and Syn.Enum. All are defaulted, and variants is unchanged.
• RustQ.Syn.Index.structs/1, struct/2, and struct!/2.
• RustQ.Rust.AST.PatternBuilder.tuple/1, for the existing PatTuple node.
• Fix: a literal atom in a typespec (required(:kind) => :if_node) is typed as Atom. It used to be emitted as a Rust type named if_node.
• Fix: keyword field access (value.type), keyword struct literal and pattern fields, and keyword macro arguments now render as raw identifiers (r#type) in both renderers. Previously field access failed to render. The existing r#type spelling keeps working.

Verification

• mix ci passes: 802 tests, credo, dialyzer, ExDNA, Reach, format, rust.fmt, rust.check, rust.clippy, rustq.gen --check, corpus, and templates.
• Focused tests cover each encoding rule, the per-type options, atom derivation, Syn.Index input, and every error path. Direct render tests cover the keyword fix and PatternBuilder.tuple/1.
• The public consumer fixture generates encoders for a small IR with Box, Vec, Option, HashMap, a keyword field, a newtype, and all variant kinds. Its package test now also runs cargo clippy -- -D warnings, which is clean.

First real consumer

vize_ex's Vapor IR encoders were generated with this branch, replacing its handwritten vapor_ir encoders. The generated code compiles clippy-clean with -D warnings. Across 18 templates covering every IR operation, the output kept every existing key and value, with 0 removed keys and 0 changed values, and it now also includes the IR fields the handwritten code dropped. The tag collision check and transparent: came out of that migration.