feat: detect weak role hierarchies in Soroban contracts

[Joeloo1]

Summary

• Adds detectWeakRoleHierarchies() detector in rules/stellar/access-control/
• Flags role-assignment functions (grant_role, add_admin, set_role, etc.) that lack a superior-authority guard, allowing any authenticated caller to escalate privileges
• Uses brace-counting to scope each function's analysis precisely, avoiding false positives from adjacent functions

Test plan

• Weak functions (only require_auth, no role-tier check) are flagged
• Functions guarded with assert_admin, only_admin, or admin.require_auth are not flagged
• Contracts with no role-assignment functions return detected: false
• Fixture stellar-weak-role-hierarchy.json matches detector output

Closes #358

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