Delete @deepseek-ai/dsh-invariants, every package's ./invariant companion
and its tests, the invariant gates and Vitest setup, the scoped-event
resolver generator, and the INVARIANT-coded listener rethrow paths.
Update docs, READMEs, Agent Notes, and add an upgrade guide.
The document Code preview and the Host read card each owned an
extension-to-language table that had drifted: the preview table had 13
extensions the read card lacked, and the two returned different language
ids for the same suffix. Both now resolve through the new zero-dependency
@deepseek-ai/dsh-util-code-language, which also adds common Windows,
config, markup, and language extensions. Every added grammar stays in the
lazy set; the boot grammars are unchanged.
Fixes#4822
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