[memx13]

Fixes #101.

ChunkHeader._load_strings and ._load_templates follow each hash-bucket's on-disk next_offset chain with no protection against an offset that repeats. Since next_offset is read straight from the file, a crafted chunk with a cyclic chain (A.next_offset -> B, B.next_offset -> A) makes the while ofs > 0: loop run forever, so parsing an untrusted .evtx hangs the process (CWE-835).

This change records the offsets already visited while walking a bucket and breaks out of the loop when one repeats. Well-formed files follow a strictly terminating chain, so their behaviour is unchanged; the strings/templates dict is still keyed by offset exactly as before.

Test

tests/test_cyclic_offsets.py builds a single chunk whose string (and template) bucket points at a two-node cycle and asserts the loader terminates. The traversal runs in a short-lived daemon thread, so an unfixed loop surfaces as a failed assertion instead of wedging the whole suite.

Verified:

• on current master both new tests fail (the loaders never return);
• with this change pytest is green — 24 passed, including the 2 new tests;
• black, isort and ruff (line-length 120) are clean on the changed files.