docs: as-built reference for the statement OCR capture

Gives receipt capture the same treatment policy OCR just got: a doc that
records what is in the code, separate from the spec that records what was
designed. RECEIPT_CAPTURE_SPEC.md §2 had accumulated three BUILT notes
totalling ~120 lines of findings, which is the right place for the
evidence but the wrong place to look up how the matcher picks a column.

docs/STATEMENT_OCR.md covers the pipeline, the OCR seam and its
text-layer-first rule, all eight parsers and the ordering constraints
between them, the matcher's two governing rules and the scopedRefField
table, confirm-through-BillingService, the learning write-back, and the
API surface.

Weight goes to the things that are load-bearing and invisible from the
code shape: brand detection must run to completion before layout because
Tijuana bills predial and zona federal off the same treasury header;
scopedRefField is exported because three call sites must agree or a
reference gets learned into a column nothing searches; FEDERAL_ZONE's
accountNumber holds a peso amount, so it fails the null-guards as well
as the lookup; a misread `$` is the dangerous failure, not a missing one.

Also records that CFE/CESPT/Telnor have no unit suite — they predate the
gas/predial extension and were only verified end to end.

Cross-linked from the spec, POLICY_OCR.md, PLAN.md, README and RESUME.md.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

RESUME.md

@@ -447,6 +447,11 @@ for what's actually next.
 
 ## Statement OCR intake (`/recibos`) — DONE 2026-08-01
 
+> As-built reference: **`docs/STATEMENT_OCR.md`** (written 2026-08-02) — the
+> parsers, the matcher's scoped-field rules, confirm/learning semantics and the
+> API surface. `docs/RECEIPT_CAPTURE_SPEC.md` §2 stays the design and the
+> measured evidence. This section is the session record of building it.
+
 Plan step 11 §2 (`docs/RECEIPT_CAPTURE_SPEC.md` §2). The last big utilities
 feature: staff scan the month's utility bills and the machine proposes customer
 + amount per page, instead of keying 300+ statements per company by hand. Built