Ligatures at the start of words break word selection

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Hi y'all.

I was trying out the example vello_editor and noticed some strange behavior: Most words are selected correctly on double-click, except the word `finibus`. In that case on double-click did not include the first letter of the word.

### How to reproduce:

* write `finibus` into the editor. alternatively `fixxxx` or `flxxxxx`
* try selecting the text with a double-click

### Here is what I have discovered so far:

* It's only a problem with that word, I tried selecting all other words in the example and it works fine.
* The problem is reproducible by typing finibus into another part of the editor and trying to select it again
* It seems to be a problem with the character sequence `fi` specifically, but I also noticed it on `fl` while testing
* This seems to only happen if the word **starts** with the characters, not if it's appears in or at the end of the word.
* The behavior does not appear with `Fi`
* Double-clicking on the `f` only selects that character

### Tested with:

* Fedora 41, last updated on 2024-04-13
* Rust 1.86.0, both in debug and on release
* commit: d53a05820799a88b23a9712d9d026378a2699403

Start by reproducing the selection problem in the vello_editor example with words beginning with `fi` or `fl`, using the reported commit as a reference point. Trace how double-click word selection handles ligature-shaped character sequences, and consider the issue done when the initial character is included without breaking selection for other words or uppercase `Fi`.

Written by the indexing model from the issue text.