fix: enter reentrancy guard before validation

[xtep103]

Summary

Move enter_reentrancy_guard() earlier in the Tholos v2 entrypoints
assert_outcome, dispute, register, and withdraw, so the guard is
acquired immediately after authentication and before validation or other
state-changing work.

This is a defensive reordering only; no validation or transfer logic was
changed.

Test plan

• cargo fmt --check
• git diff --check
• cargo test — could not complete locally because the Windows MSVC
  linker (link.exe) is unavailable
• Manually verified that the existing exit_reentrancy_guard() calls
  remain in place

Closes #132

[collinsezedike]

@xtep103 closing this in favor of #151, which you opened first for the same issue (#132) and which is the more complete fix, it replaces check_reentrancy_guard cleanly in withdraw() and adds the regression test the issue asked for. This PR's version of withdraw() leaves the old check_reentrancy_guard() call in place and adds enter_reentrancy_guard() right after it, so the guard ends up checked twice for no reason, and it doesn't add any test coverage. Let's continue on #151.