Turn non-breaking space (and other whitespace) into a space

Nobody has claimed this yet.

LLM contribution: Fable for investigation and to generate test.

If a font is missing non breaking space (U+00A0), HarfRust shapes them in the same way as a space (if the font supports that). This also applies for some different space characters (Claude claims U+2000..=U+200A, U+205F, but not U+3000 - https://crbug.com/1193282).

I believe this is a Parley level font selection question, rather than a Parley Core (soon to be Engine) thing?

Reproduction failing test (Fable authored, based on our Roboto not having NARROW NO-BREAK SPACE):
```rust
#[test]
fn space_like_char_missing_from_font_should_not_trigger_fallback() {
let mut env = TestEnv::new(test_name!(), None);

let text = "a\u{202F}b";
let mut builder = env.ranged_builder(text);
builder.push_default(StyleProperty::FontSize(2048.0));
let mut layout = builder.build(text);
layout.break_all_lines(None);

let num_runs: usize = layout.lines().map(|line| line.runs().count()).sum();
assert_eq!(
num_runs, 1,
"U+202F was itemized to a fallback font instead of using Roboto's space glyph"
);
assert_eq!(layout.width(), 2516.0);
}
```

Start with the provided space_like_char_missing_from_font_should_not_trigger_fallback test, using TestEnv, ranged_builder, and layout itemization as the entry points. Run the failing test and inspect how U+202F is assigned to a font. Done means the test passes with one run and the expected width, without causing fallback for the covered space-like characters.

Written by the indexing model from the issue text.