Fix guesst() sigEmax label and refactor internal model handling

[bbnkmp]

Bug fix:

• guesst(): the sigEmax steepness parameter returned for 3+ dose-response pairs was mislabeled "delta" instead of "h", inconsistent with the 2-pair branch and the rest of the package.

Internal refactoring (no changes to function inputs/outputs):

• Centralize built-in model metadata: add internal builtInMods (model names) and nParMod (parameter counts) in drmodels.R and use them in fitMod, fitMod_helpers, maFitMod, MCPMod, Mods_helpers (checkEntries) and optDesign_helpers (nPars), removing several hard-coded copies. bFitMod keeps its own C-coupled ordering (now documented).
• Deduplicate the lg2() and xlogx() helpers: define them once in drmodels.R and drop the inline copies in sigEmaxGrad, betaMod and betaModGrad.
• Replace 1:length(x) / 1:nrow() / 1:ncol() idioms with seq_along() / seq_len() across the package for zero-length robustness.

All 596 testthat tests pass (0 failures, 0 warnings).