[Docs] Add payment-distributor design spec

[Jayrodri088]

Complexity / points: Medium (150 points) - maintainer label: points-150-medium

PR must include:

• Closes #17 in the PR description
• Tests added or updated where behavior changes
• Docs updated if public API, lifecycle, or deploy steps change
• No unwrap() in production Soroban contract paths (project convention)
• cargo test passes locally; match CI (cargo fmt, cargo clippy -D warnings) when touching Rust

Problem

payment-distributor is currently a placeholder contract, and the repo does not contain a clear written design spec describing:

• what inputs it requires to distribute payments
• what distribution algorithm is expected (e.g., investor split, fee handling)
• how it should interact with invoice-escrow and invoice-token

Without this, contributors may implement incompatible logic.

Proposed change

Create a lightweight design spec document for payment-distributor.

Acceptance criteria

• Add a design doc under docs/ (e.g. docs/payment-distributor-spec.md)
• Document the intended distribution flow at a high level (sequence of calls)
• Document the intended contract API (function list with parameters/return types)
• Document state/events expectations (what gets stored; what events get published)
• Include an explicit Γò¼├┤Γö£ΓòóΓö¼├║╬ô├╢┬ú╬ô├╢├⌐╬ô├▓┬╝Γö£Γöñ╬ô├╢┬ú╬ô├«├ë╬ô├╢┬╝╬ô├▓┬ÑΓò¼├┤Γö£Γòó╬ô├▓┬Ñ╬ô├╢┬ú╬ô├╢├▒assumptions / open questionsΓò¼├┤Γö£ΓòóΓö¼├║╬ô├╢┬ú╬ô├╢├⌐╬ô├▓┬╝Γö£Γöñ╬ô├╢┬ú╬ô├«├ë╬ô├╢┬╝╬ô├▓┬Ñ^] section if something is undecided
• Link from payment-distributor README and from docs/API.md (once it exists)

Dependencies

Recommended as a prerequisite before large implementation work in [Feature] Implement payment-distributor baseline (replace stub).

[Jayrodri088]

Deferred (Wave 1 scope)
Maintainers are focusing this wave on implementation: escrow, invoice-token lifecycle, settlement/payment flows, payment-distributor, tests, and security bugs — aligned with decentralized invoice factoring on Stellar (liquidity for sellers, yield for investors, on-chain escrow/settlement).

Documentation issues will be reopened or recreated in a later wave. If you already started work, comment here and we can link a follow-up issue.