test(splitter): balance returns 0 for unknown split or token (#99)

[jadonamite]

Summary

Adds a test asserting that Splitter::balance returns 0 for a non-existent key, documenting the intended read-of-missing behaviour. Closes #99.

balance(env, id, token) reads DataKey::Balance(id, token) from persistent storage and falls back to .unwrap_or(0), so an unknown split id or an unknown token yields 0 rather than an error. This test pins that contract.

Test

balance_returns_zero_for_unknown_split_or_token in contracts/splitter/src/test.rs:

• Unknown split id — queries balance for an id that was never created and asserts 0.
• Known split, unknown token — creates a split, deposits token X, then asserts balance for a never-deposited token Y is 0, while token X still reads back its real balance (proving the zero is specific to the unknown key, not a blanket default).

Acceptance criteria

• Test asserts 0 for unknown id and unknown token.

No contract code changed — no_std and typed errors are untouched. cargo fmt --check clean; test added under the existing suite.