Add rate-limited ERC-20 transfers sponsoring

[thaarok]

TokenTransferSubsidies - Rate-limited sponsoring of transfers for everybody

• limited txs per token per day
• whitelisted ERC-20 tokens, native tokens if whitelisted

SenderProjectSubsidies - Rate-limited sponsoring of transfers of whitelisted senders

• limited txs per project (group of senders) per day
• whitelisted ERC-20 tokens, native tokens if whitelisted (single whitelist for all projects)

[Copilot]

Pull request overview

This PR extends the gas-subsidy system to support rate-limited ERC-20 transfer sponsorship via a delegate “extension” contract. It changes the SubsidiesRegistry sponsorship selection API to return a mode + payload (fund-based vs tracked) and introduces a track() callback path for sponsorships that don’t deduct from a fund directly.

Changes:

• Update ISubsidiesRegistry.chooseFund to return (mode, payload) and add track(trackingId, fee) to support tracked sponsorship.
• Add SubsidiesRegistry delegate support (delegate, setDelegate) to forward chooseFund/deductFees/track when the base registry can’t sponsor.
• Add FreeTransfersSubsidyExtension implementing a leaky-bucket daily limit for ERC-20 transfer(...) calls, plus comprehensive tests.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
contracts/sfc/SubsidiesRegistry.sol	Adds delegate forwarding, new track() entrypoint, and changes chooseFund return shape to support tracked sponsorship.
contracts/sfc/FreeTransfersSubsidyExtension.sol	New UUPS extension contract implementing rate-limited ERC-20 transfer sponsorship via tracking IDs.
contracts/interfaces/ISubsidiesRegistry.sol	Introduces subsidy mode constants, updates chooseFund signature, adds track() to the core interface.
contracts/interfaces/ISubsidiesExtension.sol	New interface for delegate extensions (chooseFund/deductFees/track).
test/SubsidiesRegistry.ts	Updates tests for (mode, payload) return and adds a storage-layout stability check for existing slots.
test/FreeTransferSubsidyExtension.ts	New tests covering free-transfer sponsorship, leaky-bucket behavior, and access control.
ignition/modules/FreeTransferSubsidyExtension.ts	Adds an Ignition deployment module for the extension (currently has a contract-name mismatch).

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