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>

docs/POLICY_OCR.md

@@ -233,9 +233,10 @@ signal at all (which must yield no provider rather than a bad guess).
 
 ## Related
 
-- [`RECEIPT_CAPTURE_SPEC.md`](RECEIPT_CAPTURE_SPEC.md) §2 — the utility
-  statement pipeline this was lifted from, and the origin of three rules the
-  policy parser applies: detect the provider by brand before layout, only
+- [`STATEMENT_OCR.md`](STATEMENT_OCR.md) — the utility statement pipeline this
+  was lifted from, as built. [`RECEIPT_CAPTURE_SPEC.md`](RECEIPT_CAPTURE_SPEC.md)
+  §2 is its design and the measured evidence behind it. Between them they are
+  the origin of three rules the policy parser applies: detect the provider by brand before layout, only
   ever apply the Tesseract digit-confusion map (`O→0`, `S→5`, `B→8`, …) to
   fields known to be digits, and parse amounts by separator *position* rather
   than assuming `,` is thousands.