feat(registry): add per-entrypoint gas snapshot for registry [b#028]

[SINCEO2]

Description

This is a smart-contract issue for the GrantFox FWC26 campaign. Snapshot CPU/memory per call on registry — CI fails on >5% regression.

Requirements and Context

• Implement per the description above
• Add focused tests for the change
• Document any API/visible changes
• Adhere to repo's lint and code style
• Must be secure, tested, and documented

Changes

• [ADD] contracts/registry/tests/gas_snap.rs — per-entrypoint CPU/memory snapshot tests for every public registry entrypoint:

  Entrypoint	Auth required
  init	✓
  register_offering	✓
  register_offering_with_gate	✓
  is_offering_registered	✗
  registered_count	✗
  get_offering	✗

  Each test emits a machine-readable JSON line harvested by scripts/gas-regression.sh and compared against contracts/.gas-baseline.json, so CI fails on any >5% CPU or memory regression.

• [MODIFY] scripts/gas-regression.sh — harvest the new callora-registry snapshot metrics.

• [MODIFY] contracts/.gas-baseline.json — record initial registry measurements.

Suggested Execution

1. Fork the repo and create a branch
2. Implement changes touching:
   • contracts/registry/tests/gas_snap.rs
   • scripts/gas-regression.sh
   • contracts/.gas-baseline.json
3. Test and commit; open a PR with Closes #<issue-number> in the body

Guidelines

• Minimum 95% test coverage with cargo test
• require_auth on every state-changing entrypoint
• Overflow-safe math; no unwrap() in production paths
• Clear NatSpec-style /// rustdoc
• Timeframe: 96 hours

Verification

cargo test -p callora-registry --test gas_snap
✅ 10/10 passed

cargo test -p callora-registry
✅ all suites pass (2 + 5 + 10 + 15 + 7)

Closes #853

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