[andreas-fahrecker]

Summary

Adds support for arbitrary intersection thresholds using Tailwind's
bracket syntax, e.g. intersect-[0.3], alongside the existing
intersect-half and intersect-full modifiers.

Motivation

Currently the only threshold options are intersect-half (0.5) and
intersect-full (0.99). Any other threshold requires forking the
Observer script. This PR lets users pick any threshold between 0 and 1
without touching JS, the same way Tailwind already handles arbitrary
values elsewhere (opacity-[0.3], top-[117px], etc.).

Changes

• Observer script: _getThreshold() now falls back to parsing
  intersect-[<value>] off the element's class list when no named
  threshold (intersect-half / intersect-full) matches. Parsed
  values are clamped to [0, 1].
• README: documents the new modifier under a "Custom threshold"
  section, next to intersect-half / intersect-full.

Backwards compatibility

Fully additive — intersect-half, intersect-full, and the default
Observer.start() behavior are unchanged. No breaking changes.

Testing

Tested locally in an Astro project (not the repo's demo page):

• intersect-[0.3], intersect-[0.75], intersect-[1] trigger at
  the expected scroll positions.
• Out-of-range values (e.g. intersect-[1.5]) clamp to 1 instead of
  silently failing.
• Existing intersect-half / intersect-full classes still resolve
  correctly (named thresholds take priority over parsing).

Open question for maintainer

Happy to rename intersect-[value] to something more explicit (e.g.
intersect-threshold-[value]) if you'd prefer it to be more
explicitly named. Wanted to keep the first pass minimal and get your
take before bikeshedding it further.

[heidkaemper]

First of all, thanks for the idea and your contribution!

I'm going to close this PR, but the feature itself made it in. I've implemented the modifier myself, with a few things that differ from your original version:

• To answer your open question: I kept the short intersect-[0.3] syntax
• The value is now read from element.classList with an anchored pattern instead of matching against the full className string
• The named thresholds stayed plain conditions rather than a lookup object, to keep the file consistent with the rest of the codebase
• Added tests covering all of this
• I also added an intersect-margin-[…] modifier, which should pair nicely with the custom threshold.

All of this will be in the upcoming 3.0 release.