docs: rustdoc improvements for InvoiceHot, InvoiceStatus, SplitRule, StorageKey

[lonerthefirst3-sudo]

Title

docs: rustdoc improvements for InvoiceHot, InvoiceStatus, SplitRule, StorageKey

Body

This PR is documentation-only and touches contracts/split/src/types.rs and contracts/split/src/storage_keys.rs.

• InvoiceHot in types.rs already had a /// comment, but lacked a # Design section. Added one explaining why hot fields live in instance storage (single O(1) extend_ttl covering all invoices) versus the persistent-storage cold fields (InvoiceCore / InvoiceExt / InvoiceExt2), and the upgrade-wipe trade-off that comes with instance storage.

• InvoiceStatus::to_u8 and InvoiceStatus::from_u8 in types.rs now have # Examples doc tests demonstrating the round trip (Released.to_u8() == 1, from_u8(1) == Released), matching the crate's split::types module path so cargo test --doc exercises them.

• SplitRule in types.rs now documents each variant (Fixed, Percentage, Tiered) with a worked numeric example matching the actual release-time computation in lib.rs (e.g. Percentage(3_000) on funded = 10_000 yields 3_000; Tiered(5_000, 2_000) on funded = 8_000 yields 1_600 but 0 on funded = 4_000).

• StorageKey in storage_keys.rs now has an expanded /// doc comment on the enum itself explaining Soroban's 50-variant #[contracttype] limit, why new keys must be appended rather than inserted/reordered (XDR encodes variants positionally), and that a fifth enum should be added if this one nears the cap.

Closes #635
Closes #636
Closes #637
Closes #638

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