Engine: Add client slot mode that parks untaken branches

[marcoroth]

This pull request adds a client slot mode, which sends the browser the markup of the branches a request did not take, parked in a <template> that names its own region. One of them can then be built later without asking the server for markup the page was already given.

<%# herb:slots client %>

<%# herb:slots server %> stays the default and emits nothing beyond the markers, so nothing about a branch that did not render reaches the page.

What gets parked is decided while rendering, not while compiling. A branch that rendered is on the page already, so it is not sent twice, and a template whose every branch ran parks nothing at all. Herb::Engine::SlotStatics prints the markup a branch would render with everything dynamic left out, which is what a client needs to build it: the parts that never change, and the positions of the parts that do.

<template data-herb-region="app/views/posts/_card.html.erb:aaaaaaaa">
  <!--herb-branch:0:1--><b>Hello <!--herb-slot:3--><!--/herb-slot:3--></b>
</template>

Three smaller changes come with it. A conditional whose branches lay out the same way collapses into one set of slots, since which branch ran only changes what those slots hold. An element-anchored slot names the attribute it writes, so its name no longer has to be carried separately. And a template can be named by a digest of its path instead of by the path itself, because the markers go out with the page and a view tree says more about an application than its pages do.

Slots are experimental and the marker format is not stable yet.

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☁️ Nx Cloud last updated this comment at 2026-08-18 03:25:33 UTC

[github-actions]

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_{🌱 Grown from commit 9eeae6f}

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[pkg-pr-new]

npx https://pkg.pr.new/@herb-tools/formatter@2268

npx https://pkg.pr.new/@herb-tools/language-server@2268

npx https://pkg.pr.new/@herb-tools/linter@2268

commit: 9eeae6f