Make create_for_owner! idempotent and race-safe

[rameerez]

Summary

Makes Wallet.create_for_owner! idempotent and race-safe, and cleans up the gemspec for release.

Changes

Idempotent wallet creation

• Returns existing wallet if one already exists for owner + asset_code
• Handles race conditions with rescue for RecordNotUnique/RecordInvalid
• Normalizes asset_code before lookup/create
• Extracts initial_balance_credit_attributes for subclass overriding (used by usage_credits)

# Now safe to call multiple times
wallet = Wallets::Wallet.create_for_owner!(owner: user, asset_code: :coins, initial_balance: 100)
same_wallet = Wallets::Wallet.create_for_owner!(owner: user, asset_code: :coins, initial_balance: 200)

wallet.id == same_wallet.id  # => true
wallet.balance               # => 100 (initial balance only applied once)

Gemspec cleanup

• Use git ls-files for cleaner file list
• Exclude dev directories (.claude, .cursor, .github, test, etc.)
• Add Rails version ceiling (< 9.0)

Test plan

• 66 tests, 252 assertions, 0 failures
• New test for idempotent creation behavior
• Gem builds cleanly

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