feat: claim_drip read-only view

[yunus-dev-codecrafter]

closes #716

Description

Read-only view returning unsettled accrual (vested minus released) per stream.

This PR implements the claim_drip entrypoint — a convenience alias for withdrawable — alongside focused unit tests and documentation.

Changes

Smart Contract (contracts/contracts/streampay-stream/src/lib.rs)

The claim_drip entrypoint already existed at lib.rs:1639 as a read-only view that returns the unsettled accrual for a given stream_id. This PR adds a dedicated #[cfg(test)] mod claim_drip_test with four focused tests:

• claim_drip_returns_zero_before_start_time — verifies 0 before accrual begins
• claim_drip_returns_half_at_midpoint — verifies linear accrual at 50%
• claim_drip_decreases_after_withdrawal — verifies drip decreases by withdrawn amount
• claim_drip_nonexistent_stream_returns_not_found — verifies Error::NotFound for missing stream

All tests follow the existing cancel_stream_test pattern (setup(), addresses(), token_and_client()).

Documentation

• contracts/contracts/streampay-stream/README.md: Added claim_drip to the entrypoints table (non-mutating, no auth required, alias for withdrawable).
• docs/contract-smoke-tests.md: Added happy-path and error-path smoke test entries for claim_drip.

The inline doc comment on claim_drip (already present) was reviewed and is accurate.

Testing

• Contract tests pass: cargo test --lib claim_drip (requires Visual Studio C++ toolchain)
• Lint: cargo clippy
• No new dependencies added

All 4 new tests exercise both happy-path and error-path:

Test	Coverage
claim_drip_returns_zero_before_start_time	Early-time edge case
claim_drip_returns_half_at_midpoint	Midpoint linear accrual
claim_drip_decreases_after_withdrawal	Post-withdrawal consistency
claim_drip_nonexistent_stream_returns_not_found	NotFound error path

Security

This is a read-only view — it never mutates state, never requires auth, and is unaffected by the global pause flag. No security impact.

Checklist

• Implementation matches the description
• Focused tests added
• Documentation updated (README + smoke test doc)
• No secrets or keys committed
• Code follows repo style and conventions
• No new dependencies added

Related

• Issue: Add a claim_drip view to read accrued-but-unsettled (v4 patch) #716
• Branch: feature/claim-drip-view

[greatest0fallt1me]

Closes #716