Feat: New Account Index Enforcement

[0xLeo-sqds]

Implements vault index restriction to control which sub-accounts (vaults) can be used for transactions.

Overview

• User accounts (0-250): Must be unlocked sequentially via increment_account_index instruction
• Reserved accounts (251-255): Bypass validation, always available

Program Changes

New Instruction: increment_account_index
• Increments account_utilization field by 1, unlocking the next vault index
• Callable by any signer with Initiate, Vote, or Execute permission
• Cannot exceed index 250 (because that's for reserved account)

New Errors:
• AccountIndexLocked - when trying to use a vault index that hasn't been unlocked
• MaxAccountIndexReached - when trying to increment beyond 250

Validation Enforcement:
• transaction_create.rs - validates account index when creating async transactions
• transaction_execute_sync.rs - validates for Settings-based sync transactions
• transaction_execute_sync_legacy.rs - validates for legacy sync transactions

Constants (in state/settings.rs):
• FREE_ACCOUNT_MAX_INDEX = 250
• RESERVED_ACCOUNT_START = 251

Tests

8 new tests:

incrementAccountIndex.ts:
• Increment successfully (0→1)
• Increment multiple times (0→3)
• Non-signer cannot increment
• Proposer/Voter/Executor can increment (permission tests)
• Cannot increment beyond max index 250

transactionSynchronous.ts:
• Sync transaction with locked index fails
• Reserved index (251) bypasses validation

transactionCreateFromBuffer.ts:
• Creating transaction with locked index fails

[0xLeo-sqds]

Additional changes in commit 5f93ea4:

Rust (Program):
• payloads.rs: Added validate_account_indices() method to PolicyCreationPayload that extracts account indices from each policy type and validates them against settings
• settings.rs: Added validate_account_indices_unlocked() helper + calls to validate indices during PolicyCreate and PolicyUpdate execution

Tests:
• policyCreation.ts: Added increment loops to unlock indices 1-3 for existing test + new test for AccountIndexLocked error on policy create
• policyUpdate.ts: Added increment loops + new test for AccountIndexLocked error on policy update
• tests/index.ts: Enabled policyCreation and policyUpdate test suites

While running some tests caught a regression on the SDK that i created in the last PR so here's the SDK changes:
• fix-smallvec.js: Fixed CompiledHook.instructionData to use u16 length prefix (was incorrectly u8)
• CompiledHook.ts: Regenerated with correct smallArray(beet.u16, beet.u8)
• closeTransactionBuffer.ts + IDL: Regenerated (unrelated upstream changes)