fix(css): strip .module from CSS output filenames

[sxzz]

• This PR contains AI-generated code, but I have carefully reviewed it myself. Otherwise, my PR may be closed.

Description

When tsdown processes CSS module files (e.g., Component.module.css), the output CSS retained the .module suffix. Downstream bundlers would then re-process it as a CSS module, causing styles to be lost. This adds a toCssFileName() utility that strips .module when converting JS chunk filenames to CSS filenames, applied at all 3 sites where this conversion happens.

Linked Issues

Fixes #864

Additional context

The fix uses a single regex (?:\.module)?(\.[cm]?js)$ → .css which handles both module and non-module filenames. All existing tests pass unchanged since they use JS entries that import .module.css files (so CSS filenames derive from the JS chunk name, not the CSS module filename).

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commit: dd21461