docs: add 837p reference note

docs/reference/837p.md

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+# 837P — Professional Claims (005010X222A1)
+
+The 837P transaction set carries professional (outpatient) healthcare claims
+from a billing provider to a payer. Cyclone parses the segments it needs to
+produce a structured `ClaimOutput` and validates against CO Medicaid rules.
+
+## File format
+
+- Extension: `.txt`
+- Encoding: ASCII (UTF-8 also accepted)
+- Delimiters (declared in `ISA`): `*` element, `:` component, `~` segment, `^` repetition
+
+## Envelope
+
+| Segment | Purpose |
+|---|---|
+| `ISA` / `IEA` | Interchange envelope (sender ↔ receiver) |
+| `GS` / `GE` | Functional group envelope |
+| `ST` / `SE` | Transaction set envelope (837) |
+
+## Loops
+
+| Loop | Contents |
+|---|---|
+| 2000A | Billing provider hierarchy (NM1*85) |
+| 2000B | Subscriber hierarchy (NM1*IL) |
+| 2300 | Claim (CLM, HI, NM1, DTP, REF…) |
+| 2400 | Service line (LX, SV1, DTP) |
+
+## Segments Cyclone parses
+
+- `NM1`, `N3`, `N4` — names and addresses
+- `REF` — prior auth (`REF*G1`), provider taxonomy, etc.
+- `CLM` — claim header; `CLM01` = patient control number, `CLM02` = total claim charge
+- `HI` — diagnoses (qualifier `ABK` = ICD-10 principal)
+- `LX`, `SV1` — service line + procedure code
+- `DTP` — service date (`DTP*472`)
+- `BHT` — beginning of hierarchical transaction
+
+## Segments preserved but not modeled
+
+All other segments are kept in `raw_segments` for audit but are not extracted
+into the structured `ClaimOutput`. See `cyclone/parsers/parse_837.py` for the
+full walker.
+
+## CO Medicaid specifics
+
+- Trading partner: `SKCO0` (sender) ↔ `COHCPF` (receiver) on `NM1*PR` / `NM1*40`
+- `CLM05` is a composite of three components: place of service, facility code
+  qualifier, and frequency code (in that order)
+  - `CLM05-1` = place of service (any valid CMS POS code)
+  - `CLM05-2` = facility code qualifier (`B` for CMS POS)
+  - `CLM05-3` = frequency code; must be one of `{1, 7, 8}` (1 = original,
+    7 = replacement, 8 = void)
+- `REF*G1` carries prior-authorization number when applicable
+- No 2010BA/2010CA patient loop — subscriber is the patient
+- `CLM06` (provider signature on file) and `CLM07` (assignment of benefits) are
+  typically `Y`
+
+## Validation rules Cyclone enforces
+
+Rules are defined in `cyclone/parsers/validator.py` and registered on the
+`PayerConfig` for the active payer. The default `co_medicaid()` factory uses
+`allowed_claim_frequencies={1, 7, 8}` and the full CMS POS set.
+
+| Rule | Severity | Description |
+|---|---|---|
+| `R010_clm01_present` | error | `CLM01` (patient control number) is empty |
+| `R011_total_charge_positive` | error | `CLM02` (total charge) must be `> 0` |
+| `R020_npi_format` | error | Billing provider NPI must be 10 digits |
+| `R030_frequency_allowed` | error | `CLM05-3` ∈ `allowed_claim_frequencies` |
+| `R031_ref_g1_optional` | — | `REF*G1` is informational in v1; no issues yielded |
+| `R032_clm05_2_facility_qualifier` | error | `CLM05-2` ∈ `allowed_facility_qualifiers` (e.g. `B`) |
+| `R033_clm05_1_place_of_service_code` | error | `CLM05-1` ∈ `allowed_place_of_service_codes` (CMS POS) |
+| `R050_diagnosis_present` | error | At least one diagnosis on the `HI` segment |
+| `R060_service_dates_present` | error | Every service line has a `DTP*472` service date |
+| `R070_charges_sum` | warning | Sum of service-line charges matches `CLM02` (±$0.01) |
+| `R100_payer_id_matches` | warning | `NM1*PR N104` matches the configured `payer_id` |