the BIN properties cache is dangerous

[markusicu]

If you forget to clear the ../Generated/BIN cache of property data and the data structures changed or the .txt files have changed since the last tools run, then you are looking at stale data which has the wrong values or even values incompatible with the current code.

That's why various instruction pages (build.md, pipeline.md) say that you should clear some or all of BIN.

This is error-prone, and can waste a lot of time.

TODO:

• [ ] We should turn off the BIN cache by default, so that developers don't get tripped up, and so that we can drop clearing BIN from the instructions.
• [ ] We should keep the BIN cache (via a flag) for use in CI checks which start from a clean slate and run faster with the cache.
• [ ] We should keep the BIN cache (with different contents/format?) for use in the JSPs which start from a clean slate and really benefit from the cache.

Discussion: https://github.com/unicode-org/unicodetools/pull/1116#issuecomment-2847789980