feat: Fay–Herriot measurement-error method for sample-based auxiliaries (Phase 7, v1.1.0)

[bakodramane]

Trimmed Phase 7 — Survey-based auxiliary variables (v1.1.0)

Adds one new area-level method, fh-me — the Fay–Herriot model with measurement error (Ybarra & Lohr, 2008) — for auxiliary variables that come from a large sample (e.g. an agricultural census run as a sample) rather than a full census or register, and therefore carry sampling error. Standard Fay–Herriot assumes auxiliaries are known exactly; FH-ME corrects for that. No existing method's behaviour changes except the recommender/wizard extensions described below.

=== PHASE 7 (TRIMMED) REVIEW REPORT ===
Branch:        phase/7-survey-auxiliary
PR:            (this PR)
Build:         PASS
Tests:         PASS (473 unit passed, 0 failed; 3 Playwright e2e passed, 0 failed)
Lint:          PASS

What was built:
- types/index.ts: DataAvailability.auxiliaryFromSample + hasAuxiliaryVariances;
  CatalogueEntry.requiresAuxiliaryVariances + mseMethod 'jackknife';
  UserInputs.auxiliaryVarianceVars; new 'auxiliary-variance' VariableRole
- src/catalogue/fh-me.ts (new) + registered in index.ts → 17 entries
- catalogue.test.ts: expects 17 entries; mseMethod union accepts 'jackknife'
- engine/recommender.ts: ranks fh-me first for sample-based continuous/proportion
  targets; sampling-error caveat on fh-eblup/spatial-fh/robust-fh; blocking caveat
  on fh-me when variances missing; hides fh-me for register auxiliaries
- engine/codegen.ts: {{AUX_VAR_VARIANCES_R}} + {{CI_ARRAY_BUILDER_R}} tokens
- wizard: Step3 auxiliary-source question + follow-up; Step2 variance pairing;
  Step4 badge + prominent amber caveat; Step5 variance mapping in summary
- tests: 3 new recommender scenarios; fh-me codegen snapshot; Playwright sample-aux flow
- docs: SAE-CATALOGUE.md §7 + §1 tokens; adding-a-method.md; README feature

Acceptance criteria:
- [x] All pre-existing tests still pass; new recommender, codegen, and e2e tests pass
- [x] 17 catalogue entries validate, including extended mseMethod union
- [x] FH-ME generates a runnable, commented R script using emdi::fh(method = "me")
      with a correctly assembled Ci array and no unreplaced tokens
- [x] Wizard lets the user declare sample-based auxiliaries, supply variance columns,
      and reach a downloadable FH-ME R script; standard area-level methods show the
      sampling-error caveat in that flow
- [x] npm run build, npm test, the Playwright suite, and npm run lint all pass

Known gaps or decisions deferred:
- Auxiliary↔variance pairing uses an optional 'auxiliary-variance' role matched by
  order (the spec's "keep it simple" option), with a validation message if the counts
  differ. A strict per-row paired selector was deliberately not built.
- The generated Ci array assumes zero covariance between auxiliaries (documented in a
  comment in the script), as the spec specifies; covariance columns are not collected.
- fh-me is hidden from the recommender unless sample-based auxiliaries are declared, so
  register-auxiliary flows are byte-for-byte unchanged.

Ready for review: YES

Tag v1.1.0 after merge.

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