fix: correct thiserror format string in JsonRpc error variant

[vicistar-star]

Summary

Closes #191.

Verifies and unblocks the crates/core/src/decode/ module tree so that use prism_core::decode; compiles and the module appears
in cargo doc.

Changes

• crates/core/src/decode/ — confirmed full module structure in place:
  • mod.rs — submodule declarations + decode_transaction entry point
  • host_error.rs — error classification and category parsing
  • diagnostic.rs — diagnostic event enrichment
  • report.rs — DiagnosticReport assembly from taxonomy lookup
  • context.rs — transaction context enrichment (function, args, fees, resources)
• crates/core/src/error.rs — fixed invalid thiserror format string in JsonRpc variant ({0.field} syntax is unsupported;
  replaced with named field syntax)

Verification

• cargo check --package prism-core — ✅ no errors
• cargo doc --package prism-core --no-deps — ✅ generates successfully; prism_core::decode visible in module tree

[Emrys02]

@vicistar-star fix conflict in your branch