feat: add multi-stage invoice release

[Nuwe10]

Summary

Implements creator-triggered staged fund distribution for large invoices (issue #86).

Changes

types.rs

• Added release_stages: Vec<u32> to Invoice struct — stores basis points per stage (must sum to 10 000 when non-empty)
• Added released_stages: u32 to Invoice struct — tracks how many stages have been distributed
• Added release_stages: Vec<u32> to InvoiceOptions — passed at invoice creation
• Updated Invoice::from_legacy to include new fields with zero defaults

lib.rs

• Updated create_invoice / _create_invoice_inner to accept and store release_stages
• Validates that release_stages sums to 10 000 basis points when non-empty
• Added stage_release(env, invoice_id, creator) — requires creator auth, distributes the next stage's proportion proportionally to recipients, sets status to Released on the final stage
• Updated _pay auto-release guard to block when release_stages is non-empty
• Updated all _create_invoice_inner call sites (batch, subscription, rollover, template)

test.rs

• test_stage_release_3_stages — 3-stage (30%/40%/30%) happy path verifying correct amounts per call
• test_stage_release_after_all_stages_panics — panics after all stages released
• test_stage_release_non_creator_panics — non-creator cannot call stage_release
• test_stage_release_not_fully_funded_panics — panics if invoice not fully funded
• test_create_invoice_invalid_release_stages_panics — panics if stages don't sum to 10 000

Test Results

cargo clippy: 0 warnings

closes #86

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