feat: ensure_at_rule_block/5 and AtRule.body, and fix three transform defects

[shahryarjb]

What this adds

A block at-rule — @theme { … }, @plugin "x" { … } — had no codemod. ensure_at_rule inserts a statement line and replace_rule_body works on style rules, so giving a block a body meant reaching for a regex, and re-running an installer appended a second copy.

ensure_at_rule_block/5 replaces an existing body or inserts the rule whole, so it is idempotent:

{:ok, out} = IgniterCss.ensure_at_rule_block(~s|@import "tailwindcss";\n|, "theme", nil, "--color-a: red;")

out.source
#=> ~s|@import "tailwindcss";\n@theme {\n  --color-a: red;\n}\n|

AtRule.body hands back the block's bytes verbatim, so a block can move between files without losing the comments and grouping its author put there:

{:ok, [theme]} = IgniterCss.get_at_rules(File.read!("theme.css"), "theme")
IgniterCss.Codemods.ensure_at_rule_block(igniter, "assets/css/app.css", "theme", nil, theme.body)

The body is spliced through reindent rather than rebuilt from parsed declarations — rebuilding drops comments, which is exactly what a caller moving a block would not expect.

What this fixes

corpus_invariants runs five invariants over every mutating op, but beautify and minify reprint the whole file and were never in it. Adding tests/transform_invariants.rs — output parses, is idempotent, and keeps every selector, declaration, at-rule and comment — turned up three defects that had been shipping silently:

	before	after
beautify emitted ": " for every COLON	.a:not([x])	.a: not([x])	invalid selector
beautify only kept a space after a word char	url(x) no-repeat	url(x)no-repeat	valid, unreadable
minify dropped the ; before every }	@apply x;	@apply x}	rule stops parsing¹

¹ the enclosing rule was no longer a qualified rule, so .typography vanished from list_selectors — a later codemod would simply not find it.

Each is decided from the CST rather than the text: the colon by its parent node kind (CSS_PSEUDO_CLASS_SELECTOR and its function variants versus CSS_GENERIC_PROPERTY / CSS_QUERY_FEATURE_PLAIN), the semicolon by CSS_DECLARATION_WITH_SEMICOLON.

Separately, detect_indent returned the smallest indent width present while its own doc said "the most common". One stray shallow line then decided the file's indent for every op that re-indents: a four-space stylesheet carrying a couple of two-space lines reported two, and inserted blocks came out half-indented.

Testing

pathological_selectors.css joins the corpus — :not() with a selector list, :nth-child(2n + 1), :has/:is/:where, attribute values containing braces and colons, escaped selectors, data-URIs, nested at-rules and @layer.

• 402 cargo tests, cargo fmt --check and clippy -D warnings clean
• 42 doctests + 300 Elixir tests, mix format --check-formatted and credo --strict clean

Comparison in the invariants is by meaning, not bytes: reprinting may write .a, .b where the source said .a,.b, and minifying may drop a comment — neither may change which elements match.