Update docs for postfix as cast syntax Closes #32

[binaek]

Summary

Documentation and syntax highlighting now describe postfix expr as Type casts instead of the removed cast keyword, in line with sentrie-sh/sentrie#85.

What this PR does

• Rewrites the converting-types sections on the reference pages to use postfix as, adds a short precedence note with commented examples, and updates the keyword summary on the reference index.
• Removes cast from the TextMate-style keyword list used for higher-order-related highlighting so cast is not styled as a reserved word.

Changes by area

Reference documentation

• src/content/docs/reference/types-and-values.md: coercion prose, examples, and precedence note.
• src/content/docs/reference/constraints.md: type conversion with constraints examples.
• src/content/docs/reference/index.md: casting subsection and example.

Tooling

• src/sentrie-grammar.ts: keyword regex for builtins no longer includes cast.

Review notes

• Merge or release order: ship after the compiler change (sentrie#85) so live snippets match the implementation.

Testing notes

• No automated doc tests; spot-check rendered pages if the site is built locally.

Dependency changes

• None.

[github-actions]

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