[nfisher]

Motivation

• Provide a focused API for semantic HTML assertions (DOM structure, selectors, text, and attributes) to complement existing httpassert, jsonassert, and yamlassert packages.
• Use a standards-compliant HTML5 parser and CSS selector engine rather than ad-hoc parsing to avoid brittle string comparisons and selector handling.
• Keep selector handling small and correct by pinning github.com/andybalholm/cascadia and reusing golang.org/x/net/html for parsing.

Description

• Add a new package hammy/htmlassert implementing String, Bytes, and Reader wrappers and assertions including EqualTo, HasSelector, NoSelector, SelectorCount, TextEqualTo, TextContains, AttributeEqualTo, HasAttribute, NoAttribute, and Contains.
• Implement semantic canonicalization and structural containment checks for HTML using golang.org/x/net/html and CSS selector matching via github.com/andybalholm/cascadia.
• Add tests in hammy/htmlassert/htmlassert_test.go that exercise document equality, selector queries, text/attribute assertions, fragment containment, and reader/byte variants, and add an example in hammy/htmlassert/example_test.go.
• Update go.mod to add github.com/andybalholm/cascadia and golang.org/x/net dependencies and update README.md and hammy/README.md with usage examples and package description.

Testing

• Ran unit tests for the new package with go test ./hammy/htmlassert, which executed htmlassert_test.go and the example, and all tests passed.
• Ran repository-wide tests with go test ./... and no test failures were reported for the modified code paths.

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Codex Task