feat: add invoice group all-or-nothing release

[marvs8]

Summary

Implements grouped invoice all-or-nothing release, as described in issue #33.

Changes

lib.rs:

• Storage helpers: group_key, invoice_group_key, load_group, group_all_funded
• create_invoice_group(invoice_ids: Vec<u64>) -> u64 — links invoices and maps each back to the group
• pay() auto-release now skips if the invoice belongs to a group where not all members are fully funded
• release() asserts all group members are funded, then releases every Pending member in the group; non-grouped invoices behave identically to before

test.rs:

• test_group_partial_fund_blocks_release — funds only one member, verifies release() panics with "group members not fully funded"
• test_group_all_funded_releases_both — funds both members, verifies both are Released and recipients receive correct amounts
• test_non_grouped_invoice_unaffected — verifies solo invoices still auto-release normally

Acceptance Criteria

• create_invoice_group() links multiple invoice IDs
• release() on a group member panics if any other member is not fully funded
• All group members release successfully once all are funded
• Non-grouped invoices behave identically to current implementation
• Test creates group of 2 invoices, funds both, verifies both release
• All existing cargo test pass (10/10)
• cargo clippy passes with zero warnings

Closes #33

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