refactor: ChainStore, RpcError, typed boundaries, errors module

[RaoulSchaffranek]

A bigger-picture architectural pass, following the contained cleanups in #43. Stacked on #43 (base is that branch, not main) because both heavily touch server.py — retarget to main once #43 merges. No behaviour change: all 90 tests pass and make check is clean.

This grew out of three questions: what does the dependency graph look like, which stateful components can be isolated, and how strong are the types. Four related changes:

1. errors.py — decouple the encoder from the interpreter

transaction.py raised the interpreter's NodeInterpreterError for encoding failures (sub-stroop XLM, bad strkey), which forced a transaction → interpreter import that existed only for that exception type — the one semantically-wrong edge in the module graph. A new errors.py collects the internal exceptions (NodeInterpreterError, TransactionEncodingError, RpcError); the encoder now raises TransactionEncodingError and no longer imports interpreter at all.

2. RpcError — remove union-encoded control flow

The envelope builders returned dict[str, Any] | str | None, where str secretly meant "pre-formatted JSON-RPC error" and None meant "unknown method", disambiguated at the call site with isinstance(read_only_envelope, str). They now return dict[str, Any] and raise RpcError; handle_rpc delegates to a _dispatch helper and catches RpcError once. request_id drops out of the builder signatures (it was only there for error formatting). This matches the exception-based style ledger_entries.py already used.

3. ChainStore (store.py) — isolate the stateful disk layer

StellarRpcServer conflated HTTP transport, io-dir/disk layout, and RPC orchestration, with the disk schema smeared across ~6 *_dir attributes and ~5 scattered inline json.loads((io_dir/'metadata.json')…) reads. ChainStore now owns the layout and is the sole reader/writer of state.kore, metadata.json, and the receipts/ ledgers/ events/ requests/ wasms/ files. The server holds one and asks it for receipts, ledgers, events, and the ledger counter. server.io_dir / server.state_file are preserved for callers and tests.

4. TypedDicts at the boundaries

• TxRequest / SimulateRequest — the encoder's request envelopes
• SimulateResult — the K simulate result shape
• LedgerRecord / EventRecord — the disk records the store reads/writes

interpreter.run widened to Mapping[str, Any] so typed envelopes are assignable. Functional TypedDict(...) syntax is used where keys are camelCase JSON wire names (so pep8-naming stays happy).

Deliberately left as dict[str, Any]

The read-only request envelopes (_read_only_envelope and friends) — they are built by {**base, …} spread and pattern-matched dynamically by node.md, where a TypedDict fights the idiom (spreads widen to dict[str, object], forcing casts) for little gain. Noted rather than forced.

Module graph after this change

server → store, interpreter, transaction, ledger, ledger_entries, result_xdr, scval, errors
transaction → scval, errors        (was: scval, interpreter)
interpreter → utils, errors
store → (stdlib only)

Test changes

test_unit.py now expects TransactionEncodingError from _xlm_to_stroops (was NodeInterpreterError) — a direct consequence of change #1.