Document default layout renderer alternatives

[bcomnes]

Closes#239

This PR documents how to swap DomStack's fragtml default layout renderer for the previous HTM/Preact approach using htm/preact and preact-render-to-string.

It also presents async-htm-to-string as a more direct and potentially more performant alternative for server-only layout templates that mostly combine strings and already-rendered page HTML.

The README now covers:

• How to configure browser JSX/TSX bundles now that DomStack no longer ships a default JSX runtime.
• Why Preact is the recommended JSX runtime for DomStack client bundles.
• How to alias React dependencies to @preact/compat, with a link to https://bret.io/blog/2026/simple-tanstack-query-in-preact/ for more detail.
• How to configure React when a project needs React-specific APIs or ecosystem packages.
• How to write a custom layout using htm/preact and preact-render-to-string.
• How to write a similar layout using async-htm-to-string.
• Important async-htm-to-string gotchas around standard HTML attributes, awaiting html, and rawHtml() safety.

[Copilot]

Pull request overview

Adds documentation for using async-htm-to-string as a Preact-free option for server-side HTML rendering in DomStack, addressing #239.

Changes:

• Documented async-htm-to-string as a string-rendering alternative to htm/preact + preact-render-to-string
• Included examples and highlighted key behavioral differences and gotchas (attribute naming, await usage, rawHtml() XSS risk)

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[coveralls]

Coverage Report for CI Build 28295970688

Coverage increased (+0.4%) to 92.317%

Details

• Coverage increased (+0.4%) from the base build.
• Patch coverage: No coverable lines changed in this PR.
• No coverage regressions found.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	4192
Covered Lines:	3974
Line Coverage:	94.8%
Relevant Branches:	793
Covered Branches:	628
Branch Coverage:	79.19%
Branches in Coverage %:	Yes
Coverage Strength:	75.6 hits per line

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💛 - Coveralls

[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated 2 comments.

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[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated 3 comments.

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