feat: enforce vesting cliff_secs gate before pro-rata accrual

[ugoocreates-pixel]

Closes #853

Implements vesting cliff support with a discrete cliff_secs gate before any pro-rata accrual begins, enforcing a hard zero claim before the cliff while preserving schedule arithmetic afterward.

Changes:

• Added cliff_secs: u64 parameter to the VestingSchedule struct and the vesting_register registration function.
• Added a typed VestingCliffNotReached (109) error to the VestingError enum to avoid silently returning zero when attempting to claim before the cliff.
• Updated compute_vested to strictly return 0 if the current timestamp is less than start_ts.saturating_add(cliff_secs).
• Enforced the VestingCliffNotReached check in vesting_claim prior to computing claimable tokens.
• Added comprehensive regression and boundary tests in vesting_test.rs covering happy paths, pre-cliff error emissions, and boundary conditions (exactly at start_ts + cliff_secs). Existing deployment API compatibility rules are maintained.

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