feat(tii): type-directed argument encoding to the TaggedArg wire form

[scarmuega]

What

Teaches the SDK to serialize an argument of an aggregate type (record, List, Map, Tuple, variant) into the TRP TaggedArg wire form, so trix invoke / any SDK consumer can pass complex args. New tii::encode, wired into Invocation::into_resolve_request.

One walk, not two paths

encode(param, value) is a single recursive walk over (type, value) — scalars are just the leaf cases:

• A scalar leaf renders bare at the top level (the resolver coerces it via the param's flat TIR type) and tagged when nested inside an aggregate (the resolver has no element type there).
• An aggregate always renders to its tagged structural form, recursing into elements.

So every mapped arg goes through encode; there's no is_aggregate gate. Bare top-level scalars keep the exact pre-existing wire shape, so this is back-compatible for scalar-only protocols.

Encoding rules

• Record → { "struct": { "constructor": 0, "fields": [<declared order>] } }. Field order comes from the schema's required array — tx3c emits it in source-declaration order, whereas properties is alphabetized, so required (not properties) drives positional order.
• Variant → struct with constructor = the case index in the .tii oneOf ordering.
• Map → array of [key, value] pairs; keys are string leaves (the .tii erases the Tx3 key type), pairs sorted by key for determinism.
• Reject before sending: missing/extra record field, wrong tuple arity, unknown variant case.
• unknown / utxo / anyAsset params pass through unchanged (no element types to drive encoding).

Breaking change

ParamType::Record now carries an ordered Vec<(String, ParamType)> instead of a BTreeMap (the encoder needs declared order). Added ParamType::field(name) for by-name lookup. This is the coordinated breaking SDK change; the version bump + publish are a separate release step.

Tests

• Vector-driven over the shared cross-SDK oracle (tests/fixtures/wire-vectors.json, mirrored from sdk-spec): every accept vector encodes byte-equal, every reject vector errors.
• Field-order regression (Meta { tags, level } → [list, int], not alphabetical [int, list]).
• Top-level-bare vs nested-tagged scalar rendering.
• End-to-end into_resolve_request on a real TII: meta tagged, scalars bare.

cargo test -p tx3-sdk --lib green (23); cargo clippy --lib clean.

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