feat: implement issues #833, #834, #835, #836

[Danieljyde]

#836 — Payment fallback recipient

• Add set_fallback_recipient(caller, invoice_id, recipient, fallback) entry point
• During release, if a recipient is locked and a fallback is configured, funds are routed to the fallback address instead of accumulating to UnreleasedFunds
• Storage key: fallback_recipient_key(invoice_id, recipient)
• Events: fallback_set, fallback_used

#835 — Multi-signature invoice creation (N-of-M co-signer approval)

• Add cosign_invoice(signer, invoice_id) entry point
• Add get_cosign_status(invoice_id) -> (approvals, required) entry point
• At creation, co_signers + required_signatures config is persisted for the new cosign_invoice / get_cosign_status flow
• Storage keys: multisig_required_key, multisig_signers_key, multisig_approvals_key
• Events: invoice_cosigned, invoice_cosign_complete
• Errors: SignerNotAuthorized (72), AlreadySigned (73)

#834 — On-chain invoice sentiment analysis

• Add submit_sentiment(payer, invoice_id, positive) entry point (payer must have contributed; one vote per payer)
• Add get_sentiment(invoice_id) -> (positive_count, negative_count) entry point
• Storage keys: sentiment_positive_key, sentiment_negative_key, payer_sentiment_key
• Event: sentiment_submitted
• Reuses existing AlreadyRated (24) error for double-vote

#833 — Creator bankruptcy protection

• create_invoice now panics with CreatorLocked (74) if creator has pending disputes
• raise_dispute increments creator dispute count and sets creator_lock_key=true
• resolve_dispute decrements dispute count; unlocks creator when count reaches 0
• Add get_creator_lock_status(creator) and unlock_creator(caller, invoice_id) entry points
• Storage keys: creator_lock_key, creator_dispute_count_key
• Events: creator_locked, creator_unlocked
• Error: CreatorLocked (74)

Tests: full coverage for all four features in test.rs
close #833
close #834
close #835
close #836

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