feat(sponsored): switch to B1 full-sponsorship model (no Sofia fees, explicit assets)

On the sponsored channel the pool now funds 100% of the tx cost regardless
of wallet balance. The inverse-formula deposit(3-arg) can't convey an
explicit intent, so it's disabled on this channel; callers must use
depositSponsored(termId, curveId, minShares, assets) instead.

Fees are dropped on sponsored paths (they were bookkeeping from admin's pool
back to admin's own address). accumulatedFees stays at 0, withdraw reverts
naturally via the inherited V2 guard.

msg.value is tolerantly refunded at the tail of every sponsored entry point
(nonReentrant-protected, CEI order). ExceedsMaxPerTx / InsufficientPool
guards protect the cap and pool balance upfront.

Tests rewritten end-to-end; 188 total tests pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Wieedze