fix(etl): eik_valid enforces Bulstat checksum (#195)

[StanislavBG]

Problem (#195)

eik_valid in scripts/normalize-raw.sql was only a syntactic check — digits-only, length 9 or 13, no control digit:

CASE WHEN eik_clean NOT GLOB '*[^0-9]*' AND LENGTH(eik_clean) IN (9, 13) THEN 1 ELSE 0 END

So the fake/service code „000000001" passed and became a valid eik: key, merging unrelated foreign suppliers — Elsevier + Clarivate/Web of Science + an IGB gas consultancy + a 102 EUR construction line — into one node labelled „Elsevier B. V." (eik:000000001 = 14 contracts, 45.7M EUR). Real publishers also grew a wrong-checksum twin (131106522 „Просвета" vs typo 131106552).

Fix

Add the Bulgarian ЕИК/Булстат control-digit (checksum) algorithm to eik_valid:

• 9-digit — weight positions 1..8 by 1..8; control = sum % 11. If that is 10, re-weight by 3..10; a second 10 → 0. The 9th digit must equal the control.
• 13-digit — the leading 9 digits must themselves be a valid 9-digit ЕИК, then weight positions 9..12 by 2,7,3,5 (fallback 4,9,5,7; second 10 → 0); the 13th digit must equal that control.

Invalid codes now get eik_valid = 0 and fall back to the name-based key (name:), which correctly splits Elsevier from Clarivate from the gas consultancy, and the typo twin from the real publisher. Implemented as a pure SQLite expression over the digit positions; eik_valid is computed once and the identity key references it.

Verification (live corpus, real data)

Cross-checked the SQL expression against an independent JS reference over all 13,134 currently-valid EIK nodes: 0 mismatches; 64 previously-valid nodes correctly fail the checksum.

EIK	label	old eik_valid	new eik_valid
000000001	fake/service (Elsevier merge node)	1	0
131106552	„Просвета" wrong-checksum typo twin	1	0
1027809198339	Russian ОАД (foreign 13-digit)	1	0
0018683136487	foreign 13-digit	1	0
131106522	„Просвета - София" АД (real)	1	1
2016195800141	ДГС Батак (real 13-digit branch)	1	1
8319178340012	ТУ-НИС (real 13-digit branch)	1	1
204799888	ЕМДИ Пропърти ООД (real 9-digit)	1	1

End-to-end run of the actual edited INSERT on a seeded raw_contracts: the single eik:000000001 node disappears and its four unrelated suppliers split into distinct name: nodes; valid EIKs keep their eik: node.

Scope

Checksum only. The optional alias-merge (proposal 2 in #195) is intentionally left as follow-up — it needs product input and careful handling.

Closes #195.

[StanislavBG]

Folded into #203 (checksum now lives in both normalize-raw.sql and refresh-slice.sql). Closing to keep the ETL identity work in a single PR.