feat: add graded schedule

[0xNeshi]

Towards #156

Opened as separate PR from #395 for easier and focused review just on the schedule logic.

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[Copilot]

Pull request overview

This PR adds a new graded (unequal tranche) vesting schedule implementation for the existing vesting_wallet primitive, extending the finance package with a “cumulative bps at offsets” staircase curve and a dedicated test suite.

Changes:

• Added vesting_wallet_graded schedule module exposing new / create_and_share / vested_amount / release / releasable / destroy plus view helpers.
• Implemented graded curve evaluation as a stage-based staircase using cumulative basis points with overflow guards and u128-backed mul_div.
• Added comprehensive unit tests covering constructor validation, curve shape, release behavior, destroy gating, and overflow boundaries.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
contracts/finance/sources/vesting_wallet_graded.move	New graded schedule module implementing stage-based cumulative-bps vesting on top of vesting_wallet.
contracts/finance/tests/vesting_wallet_graded_tests.move	New test suite validating graded schedule construction, vesting math, release semantics, and teardown rules.