docs: clarify safeParse callback errors

[leoafarias]

Summary

• scope the safeParse Error rethrow guarantee to constraint and refinement callbacks
• document that codec/transform decoders and safeParseAs mappers return SchemaTransformError failures for thrown values, including Error values
• keep the API reference, quickstart, and llms.txt guidance aligned

Why

PR #126 corrected the old “never throws” wording, but the replacement made the Error rethrow guarantee appear universal. The runtime intentionally wraps transform and mapper callback failures, so the documentation needs to distinguish those callback categories.

This is documentation-only and does not change runtime behavior.

Validation

• npx --yes @docs.page/cli check
• focused callback-error contract tests: 5 passed
• git diff --check origin/main...HEAD

[docs-page]

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docs.page/btwld/ack~127

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