chore(security): SHA-pin dtolnay/rust-toolchain + add top-level workflow permissions

[hyperpolymath]

Summary

Salvages an unpushed orphan commit (`bbce567` from 2026-05-23) from local main during a 2026-05-31 closeout sweep:

• SHA-pins `dtolnay/rust-toolchain@stable` → `@4be9e76fd7c4901c61fb841f559994984270fce7 # stable` in `.github/workflows/security.yml`.
• Adds top-level `permissions: read-all` to `.github/workflows/ocaml-core.yml` (was missing; baseline-deny pattern from the fleet-wide hardening pass).
• Net diff: 2 files, +4/-2.

Verification

• SHA `4be9e76f...` confirmed real via `gh api repos/dtolnay/rust-toolchain/commits/4be9e76fd7c4901c61fb841f559994984270fce7` → HTTP 200.
• Original commit GPG-signed.

Test plan

• SHA verified live
• CI passes (running)

[github-actions]

🔍 Hypatia Security Scan

Findings: 144 issues detected

Severity	Count
🔴 Critical	7
🟠 High	79
🟡 Medium	58

⚠️ Action Required: Critical security issues found!

View findings

[
  {
    "reason": "Action perpolymath/standards/.github/workflows/governance-reusable.yml@main\n needs attention",
    "type": "unpinned_action",
    "file": "governance.yml",
    "action": "pin_sha",
    "rule_module": "workflow_audit",
    "severity": "medium"
  },
  {
    "reason": "Issue in boj-build.yml",
    "type": "missing_timeout_minutes",
    "file": "boj-build.yml",
    "action": "flag",
    "rule_module": "workflow_audit",
    "severity": "medium"
  },
  {
    "reason": "Issue in cargo-audit.yml",
    "type": "missing_timeout_minutes",
    "file": "cargo-audit.yml",
    "action": "flag",
    "rule_module": "workflow_audit",
    "severity": "medium"
  },
  {
    "reason": "Issue in cargo-audit.yml",
    "type": "missing_timeout_minutes",
    "file": "cargo-audit.yml",
    "action": "flag",
    "rule_module": "workflow_audit",
    "severity": "medium"
  },
  {
    "reason": "Issue in cflite_batch.yml",
    "type": "missing_timeout_minutes",
    "file": "cflite_batch.yml",
    "action": "flag",
    "rule_module": "workflow_audit",
    "severity": "medium"
  },
  {
    "reason": "Issue in cflite_pr.yml",
    "type": "missing_timeout_minutes",
    "file": "cflite_pr.yml",
    "action": "flag",
    "rule_module": "workflow_audit",
    "severity": "medium"
  },
  {
    "reason": "Issue in codeql.yml",
    "type": "missing_timeout_minutes",
    "file": "codeql.yml",
    "action": "flag",
    "rule_module": "workflow_audit",
    "severity": "medium"
  },
  {
    "reason": "Issue in dogfood-gate.yml",
    "type": "missing_timeout_minutes",
    "file": "dogfood-gate.yml",
    "action": "flag",
    "rule_module": "workflow_audit",
    "severity": "medium"
  },
  {
    "reason": "Issue in dogfood-gate.yml",
    "type": "missing_timeout_minutes",
    "file": "dogfood-gate.yml",
    "action": "flag",
    "rule_module": "workflow_audit",
    "severity": "medium"
  },
  {
    "reason": "Issue in dogfood-gate.yml",
    "type": "missing_timeout_minutes",
    "file": "dogfood-gate.yml",
    "action": "flag",
    "rule_module": "workflow_audit",
    "severity": "medium"
  }
]

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