refactor: promote PParams shape into balius-core, slim runtime converter
The PParams wire shape — pbjson conventions from utxorpc-spec 0.17 — deserved the same architectural treatment as Tx/UTxO: defined once in balius-core, asserted with a wire-compat invariant test that proves "what we emit, 0.17 decodes," and consumed unchanged on the runtime and SDK sides. Before: the shape lived implicitly inside a 100-line imperative JSON builder in `balius-runtime::ledgers::u5c::convert::pparams_to_legacy_json` that hand-assembled `serde_json::Value` to match pbjson. The SDK had a parallel minimal PParams struct just for `coins_per_utxo_byte`. Two mostly-implicit places to keep in sync; nothing in balius-core actually owned the contract. After: - `balius_core::proto::v0::cardano::PParams` + 7 nested types (RationalNumber, ProtocolVersion, ExUnits, ExPrices, CostModel, CostModels, VotingThresholds) own the wire shape via serde + serde_with derives: camelCase keys, u64 as JSON strings via DisplayFromStr, u32 as plain numbers, default-zero / None fields skip serialization. One source of truth. - The runtime converter shrinks to a flat `convert_pparams` that flattens upstream BigInts via `unwrap_u64` and field-copies into a `balius_core::proto::v0::cardano::PParams`. serde_json::to_vec on the typed value produces the wire bytes — no imperative JSON assembly. ~140 lines removed from convert.rs. - balius-sdk drops its inline PParams in favor of `pub use balius_core::proto::v0::cardano::PParams`. - balius-core/tests/u5c_v017_wire_compat.rs (renamed from wire_compat.rs for clarity) gains `wire_compat_pparams_roundtrips_via_017` mirroring the TxOutput test: build a balius-core PParams, serialize, decode under utxorpc-spec 0.17 pbjson, assert every field. Plus `wire_compat_pparams_omits_default_fields` for the skip-default invariant. - Runtime keeps `convert_pparams_decodes_under_017_pbjson` and `convert_pparams_overflow_propagates` — they cover the upstream-side BigInt seam (balius-core can't, no upstream dep). The now-redundant `convert_pparams_omits_default_fields` is dropped. balius-core gains serde + serde_with as deps and serde_json as a dev-dep. Verified: cargo check --workspace --tests clean; balius-core 4/4 (incl. the two new PParams tests); balius-runtime --lib 8/8; e2e::faucet_claim 1/1 (full end-to-end through mock ledger). u5c-chainsync::wallet_balance remains the pre-existing stale-wasm failure. Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>
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