Changelog

Fix d3() operator precedence bug: d2[1] was always used instead of d2[i], producing wrong results when called with a vector of sample sizes (#2)
Fix stats() operator precedence bug: 1:m-1 evaluated as (1:m) - 1, producing index 0 on first iteration (affects all n=1 multivariate calculations)
Fix cchart.T2.2() operator precedence bugs: is.null(x) == FALSE patterns were fragile (5 occurrences)
Fix cchart.T2.1() incorrect UCL formula for Phase I with n=1: now uses correct beta distribution parameters per Montgomery 7th ed. (#1)
Fix cchart.S() type parameter mismatch: code checked for "c" but documentation specified "e" for exact limits
Fix cchart.Xbar_R() typo: add.stat corrected to add.stats
Fix standardized p-chart and u-chart: removed erroneous multiplication by sample size
Fix cchart.T2.1() error handling: sprintf() replaced with stop()
Fix error messages throughout: return("msg") replaced with stop("msg")
Fix typos: “Stardardized” → “Standardized”, “Shewart” → “Shewhart”, “aprroximation” → “approximation”
Add input validation (n >= 2) to d2(), d3(), c4()

Create constants.R with ALPHA, SIGMA_MULT, Q_LOWER, Q_UPPER constants
Replace 15+ hardcoded magic numbers across 7 files
Standardize T/F to TRUE/FALSE throughout
Standardize = to <- for assignment
Replace is.null(x) == FALSE with !is.null(x)
Fix table.qtukey(): remove unnecessary function definitions, add invisible() return
Pre-allocate vectors in d2() and d3() (was O(n²) with append())
Replace matrix(nrow=m, ncol=1) with numeric(m) for control limits

IQCC 0.7.1