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/RECEIPT_CAPTURE_SPEC.md

@@ -131,7 +131,12 @@ single-movement form.
 
 ## 2. PDF / OCR auto-capture
 
-> **BUILT — 2026-08-01.** Implemented and verified end to end against real
+> **BUILT — 2026-08-01.** As-built reference:
+> [`STATEMENT_OCR.md`](STATEMENT_OCR.md) — what the shipped feature does, its
+> parsers, matcher rules and API surface. This section stays the *design* and
+> the evidence behind it; go there for what is in the code today.
+>
+> Implemented and verified end to end against real
 > scanned statements. `apps/api/src/statements/` holds the module: a swappable
 > `OcrProvider` seam with a self-hosted Tesseract implementation, per-provider
 > parsers for CFE / CESPT / Telnor / gas / predial, a scoped matcher, and a