feat: enforce additive-only storage layout changes for PDPVerifier

[Chaitu-Tatipamula]

Summary

Implements storage slot safety for the PDPVerifier upgradable contract
by enforcing that storage layout changes are strictly additive,
preventing storage collision bugs during contract upgrades.

Resolves #258

What's Checked

Scenario	Detected
❌ Removing existing storage slots	Yes
❌ Changing the slot number of an existing variable	Yes
❌ Inserting new slots in the middle (shifting existing slots)	Yes
✅ Appending new slots at the end	Allowed

Changes

• tools/generate_storage_layout.sh: Script to generate
  PDPVerifierLayout.sol from contract source using forge inspect
• tools/check_storage_layout.sh: Validates that storage layout
  changes only add new slots at the end; flags destructive changes like
  removed slots, moved slots, or inserted slots
• src/PDPVerifierLayout.sol: Generated file with 17 storage slots
  (must be committed and kept in sync)
• Makefile: Added gen, check-layout, clean-gen targets for
  managing storage layout
• .github/workflows/check-storage-layout.yml: New CI workflow to
  verify storage layout on all pushes/PRs
• .github/workflows/makefile.yml: Added storage layout check to
  existing CI

Usage

make gen              # Generate/rebuild storage layout
make check-layout     # Check for destructive changes (runs in CI)
make clean-gen        # Clean generated files