Document x/ experimental packages and components.JoinAttrs in AGENTS.md

[maragubot]

What was found

AGENTS.md is the guide loaded into every AI assistant that works on this repo, since CLAUDE.md symlinks to it. Its Project Structure section had drifted from the packages that actually exist:

• The x/ experimental packages were missing entirely. x/slices landed in Add x/slices package with Map, Filter, and Reduce #284 on 2026-01-08, after AGENTS.md was written on 2025-09-24, and the section was never updated. This also put AGENTS.md at odds with the readme, which already documents gomponents/x/... as experimental.
• The components/ entry named only HTML5 and Classes, never components.JoinAttrs (added in Add components.JoinAttrs #262).

What was fixed

Added an x/ bullet and listed components.JoinAttrs, so the section matches what's on disk.

Also scoped one Code Style bullet from "Maintain backwards compatibility" to "Maintain backwards compatibility in the core library". Without that, the new x/ bullet's "may get breaking changes" would contradict it, and an assistant would read a blanket prohibition the project doesn't intend. The file already scopes this way two lines up ("No external dependencies in core library") and under Contributing Guidelines.

Review notes

Two rounds of self-review caught real errors, both now fixed: the first draft described components.JoinAttrs as joining attributes "across nodes", which implies a tree traversal it doesn't do (it only merges attributes among the child nodes passed to it); and a later draft labelled the x/slices helpers "index-aware", which is false for slices.Reduce, whose callback is func(T2, T1) T2 with no index.

Deliberately left for a later gardening pass:

• LLMs.md has the same two gaps in its own Package Structure section. Another change is in flight against that file, so this one stays out of it.
• internal/assert and internal/import are still absent from the list. They're test-only scaffolding, and that omission predates this change.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (76f04c0) to head (8625c28).

Additional details and impacted files

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##              main      #338   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            6         6           
  Lines          639       639           
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  Hits           639       639           

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