docs: SPDX headers, rustdoc coverage, audit workflow, proxy runbook

[JamesEjembi]

Changes

#789 - Apply SPDX / MIT headers to all workspace Rust files

• Added // SPDX-License-Identifier: MIT to all 148 .rs files
• Created add-spdx.sh script for automated header application

#790 - Expand Rustdoc coverage on propchain-traits to 100%

• Enabled #![warn(missing_docs)] in lib.rs
• Added doc comments to all public items lacking them across 23 module files
• Key additions: circuit_breaker.rs, eip712_permit.rs, property_token_metadata.rs, admin_multisig.rs, access_control.rs

#788 - Reduce repeat chore(ci) churn in AUDIT_LOG.md

• Modified nightly audit workflow to push to dated audit/YYYY-MM-DD branch
• Creates a PR using gh pr create with auto-merge via squash
• Results in at most 1 squashed commit per day instead of a direct commit per run

#787 - Author an Operations Runbook for Proxy Contract Upgrades

• Created docs/operations/PROXY_UPGRADE.md with:
  • Mermaid sequence diagram of the upgrade flow
  • Pre-flight checklist
  • Step-by-step CLI snippets (governance vote, timelock, proxy admin, verification)
  • Failure recovery sections
  • Rollback procedure
  • Cross-linked from proxy_upgrade_governance.md

Closes #789
Closes #790
Closes #788
Closes #787

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