docs: add high-quality processes vignette

[flaviobarros]

Summary

Part of #11.

This PR adds a second package vignette focused on high-quality processes and rare nonconformities.

Changes

• Adds vignettes/high-quality-processes.Rmd.
• Explains why classical normal-approximation p charts can be poorly calibrated for rare defects.
• Documents the use case for cchart.p(type = "CF").
• Introduces the DS-np numerical core: dsnp_prob_accept(), dsnp_arl(), and dsnp_ass().
• Explains the fractional-limit convention used by the DS-np helpers.
• Updates README to list both available vignettes.

Notes

This is documentation-only. It does not change R code, tests, or numerical behavior.

The DS-np limit search and plotting interface remain planned for future PRs under #5.