[Bug] Trunc Length in Client Hello

**Describe the bug**
While verifying the F* code, the treatment of `trunc_len` seems to be wrong.
It is treated as the length of the suffix in some places and as the length of the prefix in others.
This bears investigation.

See e.g. https://github.com/cryspen/bertie/blob/39c45ce83357da202151d7919293f3e5f83c345b/proofs/fstar/extraction-panic-free/Bertie.Tls13formats.fst#L2287

**To Reproduce**

**Expected behavior**

**Actual behavior**

**Screenshots or debug log**

**Platform (please complete the following information):**

**Additional context**

Start by inspecting proofs/fstar/extraction-panic-free/Bertie.Tls13formats.fst at line 2287 and tracing how trunc_len is used during Client Hello handling. Compare its prefix and suffix interpretations across the relevant F* code; done means its treatment is consistent and the verification no longer exposes the mismatch.

Written by the indexing model from the issue text.