Remove overhead of TLS on river point detection

[xinitrcn1]

• thread_local not recommended, stack was preferred
• Cleans up the loop logic by chaining else/if (no codegen change, just a stylistic choice)
• Stylistic choice of making the loop more clearly bounded

[Spartan322]

Could you squash this PR and mention in the commit and PR description about cleaning up the loop?

[xinitrcn1]

Done.

[Spartan322]

You'll need to run pre-commit as well.