docs: design record for TransactionPrototype

[lisicky]

Summary

Design record for TransactionPrototype. Documentation only — the implementation is #837, this PR adds no code.

The prototype is the handoff value between a transaction builder and a serializer: a fully-resolved, backend-agnostic representation of a Cardano transaction — post coin-selection, post witness collection — already in a shape friendly to serialization. It is what the builder should assemble at the end and hand to the serializer, and nothing more: no builder intent, no unresolved references, no partial state.

docs/design/tx-prototype.md records the decisions that are not obvious from reading the types:

• why it lives in mesh-common and not in either serializer package
• the Conway CDDL as the sole type authority, and why the wide fields are bigint rather than number | bigint
• what the prototype deliberately does not express (set encoding, output era format, definite lengths) and why those belong to the serializer
• why Plutus scripts are three fields instead of one
• the mesh-core-cst / mesh-core-csl asymmetry, and whisky's conformance gaps as upstream bugs rather than reasons to narrow the types
• what the decoder is for, and the round-trip asymmetries that are pinned by tests

Affect components

• @meshsdk/common
• @meshsdk/core-csl
• @meshsdk/core-cst

(documented, not modified)

Type of Change

• Documentation update (adding or updating documentation related to the project)

Related Issues

Documents the design implemented in #837.

Checklist

• My code is appropriately commented and includes relevant documentation, if necessary
• I have added tests to cover my changes, if necessary — n/a, no code
• I have updated the documentation, if necessary
• All new and existing tests pass (i.e. npm run test) — unaffected
• The build is pass (i.e. npm run build) — unaffected