feat: vesting account poc

[julienrbrt]

ref #5798

Implement vesting accounts. A small struct need to be added on genesis with the vesting account type, the vested amount and the vesting schedule. Bank will automatically verifies if a vesting account can spend those tokens.
Once the vesting schedule is over, vesting accounts are upgrade to normal accounts.

[Gno2D2]

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[moul]

A few findings not already raised in the thread (the empty-baseAcc.Coins genesis bug, missing regenerated .pb.go, and the overflow risk are already covered by @tbruyelle).

[tbruyelle]

I have a remaining comment for the proto generated files, but I dont want to block the merge for that. Approving.

[tbruyelle]

@moul OK for merging ?

[moul]

Review note — no blockers found; well-structured and well-tested. Two non-blocking warnings worth an explicit sign-off:

1. Consensus-critical surface from a poc-tagged PR. bank.SubtractCoins is on the hot path for every transfer (including the VM banker via RemoveCoin/SendCoins). The change is inert unless genesis contains vesting entries (*BaseAccount doesn't satisfy std.VestingAccount, non-vesting paths are unchanged), so no state migration is needed — but reviewers should confirm intent to land core-bank behavior from a PoC. Worth stating "no migration needed, no pre-existing vesting accounts" in the description.
2. Unrestricted-transfer bypass lets still-locked coins pay gas/storage-deposits (SendCoinsUnrestricted, auth/ante.go, bank/keeper.go). This is deliberate and tested (TestBankKeeper_VestingUnrestrictedBypass), and doesn't let a holder redirect locked funds to themselves (fees → fee collector). Fine as policy, but call it out so it's an accepted decision rather than an oversight.

Determinism checked: big.Int integer math on ctx.BlockTime(), sorted Coins iteration, no maps/floats/time.Now(). Upgrade preserves PubKey/Sequence/AccountNumber so signatures survive.

_{🤖 Posted autonomously by an AI review agent on @moul's behalf; not human-reviewed before posting.}