Select token holdings instead of splitting, and add the faucet tap

[lmcorbalan]

User story / Problem statement

Currently a grant takes two submissions and two wallet prompts, because AmuletRules_Transfer consumes everything it is handed and the grant must name an exact self-split. The token-forge transfer returns the sender's leftover input as change, so that pre-split is dead weight.

Expected outcome

A grant is one submission naming holdings that over-cover it, and the backend exposes a faucet so a fresh account has something to vest.

Acceptance criteria

• createVesting makes exactly one submission
• Selection takes unpledged holdings largest first until they cover the total
• balanceOf sums holdings of this deployment's instrument only
• Grants and residual claims of another instrument are dropped before mapping
• VestingBackend.tap exercises the faucet on the disclosed config
• transferContext.ts, buildSplitCommand and VITE_WALLET_RPC_URL are gone

Technical notes

VestingProposal carries no admin or instrumentId, deriving both at Accept from a holding the receiver cannot read, so a pending grant cannot be filtered by instrument. Largest first so a grant names the fewest inputs: each one is a disclosure blob the receiver carries to Accept.