feat: add binary codec with backward-compatible migration

[martinconic]

Checklist

• I have read the coding guide.
• My change requires a documentation update, and I have done it.
• I have added tests to cover my changes.
• I have filled out the description and linked the related issues.

Description

• Adds MarshalBinary/UnmarshalBinary to bigint.BigInt, using compact
  Gob encoding instead of the JSON fallback that *big.Int previously
  triggered in the statestore
• Migrates all balance store.Put/store.Get calls in accounting and
  chequebook (totalIssued) to use bigint.BigInt explicitly
• Fixes a backward-compat gap discovered during implementation: the old
  code stored *big.Int as an unquoted JSON decimal (e.g. 123);
  UnmarshalBinary now handles all three on-disk formats —
  Gob (new), unquoted decimal (old *big.Int), quoted string (old bigint.BigInt JSON)

Why

The statestore serializes values via encoding.BinaryMarshaler if
available, otherwise falls back to json.Marshal. Since *big.Int does
not implement BinaryMarshaler, balance values were previously stored as
unquoted JSON numbers. This change introduces a proper binary codec that
is ~2x more compact for large integers and decouples balance storage from
JSON encoding, while remaining fully backward compatible with existing
stored data.

Open API Spec Version Changes (if applicable)

Motivation and Context (Optional)

Related Issue (Optional)

Screenshots (if appropriate):

AI Disclosure

• This PR contains code that has been generated by an LLM.
• I have reviewed the AI generated code thoroughly.
• I possess the technical expertise to responsibly review the code generated in this PR.

[acud]

@martinconic are you still proceeding with this as-is? or will you add a statestore migration?

[martinconic]

@martinconic are you still proceeding with this as-is? or will you add a statestore migration?

I am working on a state migration also