Add StarGuard plot/drop helpers

[0xLaz3r]

Adds DssExecLib wrappers for StarGuard.plot and StarGuard.drop, enabling Core spells to whitelist a Star Spell for permissionless execution (and revoke it) without inlining the call.

Changes

• src/DssExecLib.sol
  • New StarGuardLike interface.
  • plotStarSpell(address _starGuard, address _starSpell, bytes32 _starSpellTag) — whitelists a Star Spell by codehash.
  • dropStarSpell(address _starGuard) — clears the currently whitelisted Star Spell.
  • StarGuard address is passed as a parameter (one StarGuard per SubProxy), matching the existing executeStarSpell(starProxy, starSpell) shape.
• src/mocks/MockStarGuard.sol — new mock with plot/drop recorders and a shouldFail toggle.
• src/mocks/MockDssSpellAction.sol — plotStarSpell_test / dropStarSpell_test wrappers.
• src/DssAction.t.sol — 5 tests: plot success, plot overwrite, plot failure, drop success, drop failure.
• README.md — documents the two new helpers in the SubDAO/Star Spells section.

Verification

• All 99 tests pass (94 existing + 5 new).
• DssExecLib runtime size: 18,549 / 24,576 bytes — well under the EIP-170 limit, so both helpers fit comfortably.

[DaiFoundation-DevOps]

All committers have signed the CLA.

[riccardopersiani]

The tests covers every line.
The size is slightly higher than reported but still safe.

[amusingaxl]

Initial nits.

[amusingaxl]

LGTM