Fix AOI export crashes: FILETIME overflow and unsafe attribute escaping

[akivabrookler]

Summary

Two real-world .ACD projects fail to export to L5X. Both crashes originate in acd/l5x/elements.py:

1. OverflowError: date value out of range — _parse_aoi_nameless converts an AOI's Created/Edited Windows FILETIME into a datetime. On some AOI nameless records the variable-length field walk drifts and reads 8 garbage bytes, yielding a FILETIME whose year exceeds datetime's 9999 ceiling. datetime(1601, 1, 1) + timedelta(...) then raises and aborts the entire export.

2. Non-well-formed L5X from unsafe attribute escaping — L5xElement.to_xml serializes attribute values with html.escape, which only handles & < > " '. When a binary field decodes to garbage (e.g. an AOI Vendor whose length/offset drifts, decoded with errors="replace"), the attribute ends up containing raw C0 control characters and even raw newlines. The emitted L5X is then non-well-formed, and strict downstream XML parsers reject it (unterminated / "open quote" attribute).

Changes

• Add _filetime_to_iso(ft): guards OverflowError/OSError and returns "" for empty or out-of-range FILETIMEs. Used for both the Created and Edited AOI dates (also de-duplicates the conversion that was inlined twice).
• Add _escape_xml_attr(value): strips XML-1.0-illegal control characters and encodes the legal-but-delimiting whitespace (TAB/CR/LF) as numeric character references, in addition to html.escape. Used for all attribute-value serialization (L5xElementBuilder.to_xml, the Communications connection name, and the Routine name).
• Add unit tests for both helpers (test/test_elements_helpers.py).

Testing

• New helper tests pass (pytest test/test_elements_helpers.py — 7 passed).
• The existing test_api.py::test_to_xml (full CuteLogix.ACD → L5X serialization, which exercises _escape_xml_attr) passes.
• Verified end-to-end against the two .ACD files that originally triggered each crash: both now export to valid, well-formed L5X.

The fixes are defensive and behavior-preserving for valid input: a valid FILETIME still formats identically, and an attribute with no illegal/whitespace characters escapes exactly as before.

[hutcheb]

Thanks @akivabrookler