improve hash file naming function

[aqk]

A "hash" file in this context is a file containing the sha256 of the input file. This patch changes the code that generates the name for that file:

• move to a function to reduce chance of errors in the future, and encapsulate details
• change transform so that input filename cannot overlap with output filename (eg. ls *hex big.hex, big_hash.hex; hash_file_name(big.hex) -> big_hash.hex)

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Note

Medium Risk
Changes the on-disk name of the treehash sidecar written alongside cached .hex outputs, which may break tooling that expects the previous *_hash.hex naming. Logic is small and covered by updated tests, but it affects build artifacts and cache behavior.

Overview
Refactors module cache rebuild output to use a shared hash_file_name() helper when writing the sha256tree sidecar for cached .hex artifacts.

Updates the sidecar naming scheme from the previous *_hash.hex pattern to a non-overlapping "<hex_filename>.hash" form, and adjusts/extends module_cache tests to validate the new naming and paths.

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