feat: Phase 3 — code-generation engine + AUX_VARS_R_VEC pre-work

[bakodramane]

Pre-work (before generateCode):

• docs/SAE-CATALOGUE.md §1: add {{AUX_VARS_R_VEC}} token (quoted R character-vector form, for column-selection contexts)
• src/catalogue/bhf-eblup.ts, reblup.ts, hb-unit.ts: rename non-standard {{AUX_VARS_R_QUOTED}} to {{AUX_VARS_R_VEC}} in all c(...) index positions

Code-generation engine (src/engine/codegen.ts):

• UserInputs and GeneratedCode interfaces per PHASES.md §3a
• buildTokenMap(): maps all {{TOKEN}} names to user-supplied values; defaults survey.csv / census.csv / 200 sims / bootstrap MSE
• substituteTokens(): global regex replace, no unreplaced tokens escape
• Stata v14 fallback: stataVersion < stataMinVersion and fallback non-null → use stataV14Fallback template, set usedFallback:true + fallbackNote
• R and Stata footers appended: MSE/RMSE interpretation + next-steps note

Snapshot tests (src/engine/codegen.test.ts, 50 assertions):

• fh-eblup: library(sae), fhsae command, all vars, no {{ tokens
• bhf-eblup: library(sae), mixed command, all vars, AUX_VARS_R_VEC quoted form
• ebp-censuseb (stataVersion:14): usedFallback true, Stata uses mixed
• glmm-binary: library(lme4), meglm command, bernoulli logit family
• Fallback boundary: v16<17 triggers fallback; v17>=17 uses full template
• Token edge cases: single var, three vars, custom N_SIM, purity check

All 430 tests pass (380 pre-existing + 50 new).