Vouch request: fix CircBuf resize state corruption

[fallintoplace]

What do you want to change?

I’d like to fix a state-consistency bug in src/datastruct/circ_buf.zig when CircBuf.resize shrinks a partially filled buffer.

After reallocating the storage, resize currently updates head and full only when the buffer grows. When a partially filled buffer is shrunk below its current length, the old metadata can remain in place, leaving head beyond the new capacity and causing len() to report more elements than the buffer can contain. A later append can then access the storage out of bounds.

Why do you want to make this change?

This change would preserve the core invariants of CircBuf after resizing: the length should never exceed capacity, indices should always remain within the allocated storage, and a buffer retaining exactly its new capacity should be marked as full.

I acknowledge that:

• I have read the Contributing Guide and understand the contribution process.
• I have read and agree to follow the AI Usage Policy.
• I wrote this vouch request myself, in my own voice, without AI generating it.

[jcollie]

!vouch