Create a read-only index that drops index files not needed for searching

### Description

Now that we have vector quantization we face the possibility of writing an index that is 5 times bigger than is needed for searching. If the index is primarily vectors and they are quantized, we will save the full-precision vectors but they may not be required at all for searching. In an architecture where indexes are written on one set of hosts and replicated to another set of hosts for searching, it is wasteful to copy all of the full-precision vectors to the searcher nodes. But Lucene doesn't have any way of distinguishing. I wonder if we could create a "write read-only index" operation that would effectively clone the existing index, dropping any data required only for indexing, and mark the index as read-only so it could never be opened for writing. This might be useful in some way for version upgrades as well?

The issue names no files, tests, or entry points. Start by surveying Lucene's index-writing and read-only opening APIs, then identify which data is needed only for indexing versus searching. Done means a defined operation can produce a smaller read-only index that cannot be opened for writing, with tests covering the resulting behavior.

Written by the indexing model from the issue text.