test: add regression tests and docs for issues #476, #478, #479, #480

[harystyleseze]

Summary

Adds three regression test files and one documentation file to address issues #476, #478, #479, and #480. No
existing source code or tests are modified.

Changes

creator-keys/tests/holder_count_unchanged_after_supply_cap_exceeded.rs (closes #476)

Regression test confirming that when a buy reverts with SupplyCapExceeded, the holder count is not incremented,
the second wallet's balance stays zero, and total supply remains unchanged. The supply cap check in buy_key
returns early before any holder count or balance mutation, locking in this invariant.

creator-keys/tests/dividend_claim_zeroes_claimable_regression.rs (closes #478)

Regression test confirming that get_claimable_dividend returns exactly 0 (not the previously claimed amount)
after a successful claim_dividend call, and that a subsequent claim attempt reverts with NoDividendClaimable.

docs/transfer-supply-cap-and-dividend-interaction.md (closes #479)

Documents four non-obvious interactions between transfer_keys and the supply cap / dividend system:

• Supply cap applies only to buy_key, not to transfer_keys (supply is invariant across a transfer).
• Dividend claimable balances for past distributions are settled before balance changes on transfer; neither party's
  claimable is unaffected by the transfer.
• Future distributions after a transfer use post-transfer balances (recipient share increases, sender share
  decreases).
• Pending claimable dividend is not carried with keys on transfer — each holder claims their own independently.

creator-keys/tests/buyback_event_new_supply_regression.rs (closes #480)

Regression tests confirming the KeysBoughtBackEvent.new_supply field equals pre-buyback supply minus buyback
amount, is zero after a full buyback of all supply, and is nonzero when supply remains after a partial buyback.

Test Plan

• cargo test --test holder_count_unchanged_after_supply_cap_exceeded — 1 test passes
• cargo test --test dividend_claim_zeroes_claimable_regression — 2 tests pass
• cargo test --test buyback_event_new_supply_regression — 3 tests pass
• No existing tests modified; no warnings in new files

Closes #476
Closes #478
Closes #479
Closes #480

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