docs(context): postcode-search entry + VocabularyMapping's second producer

Left uncommitted by the previous session; restores the "### Building parts"
heading it had accidentally swallowed.

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

CONTEXT.md

@@ -45,14 +45,22 @@ _Avoid_: customer data, manual override, landlord data
 The per-Property fact layer — one resolved fact per `(Property, Building part, component)`, where component is one of `wall_type`/`roof_type`/`property_type`/`built_form_type`. Holds a **snapshot** of the resolved enum value (a denormalised copy of the VocabularyMapping outcome at finalise time, so two Properties sharing a description can later diverge), plus the original spreadsheet text it resolved from. Materialised by the finaliser **for UPRN-matched Properties only** (v2); the resolved value is never `UNKNOWN` — the Verify step forces every `UNKNOWN` to be mapped before Finalise, and an unresolved description fails the run. See [ADR-0005](./docs/adr/0005-async-bulk-upload-finaliser.md) (table) and [ADR-0006](./docs/adr/0006-property-overrides-join-and-no-uprn-defer.md) (population).
 _Avoid_: per-property mapping, property fact, override row
 
+The Model backend consumes Property overrides at modelling time; property type (and built form when known) lets the **predict-EPC service** estimate a never-EPC'd property from surrounding homes of similar archetype.
+
 **Source row id**:
 A synthetic UUID minted per source-file row at `start-address-matching` and written into **both** the address CSV and the classifier CSV. It is the stable join key that lets the finaliser tie a row's identity (combiner output → `property_id`) to that row's raw descriptions (classifier CSV), since neither file preserves row order and `Internal Reference` is absent from the classifier CSV. See [ADR-0006](./docs/adr/0006-property-overrides-join-and-no-uprn-defer.md).
 _Avoid_: row index, internal reference (a separate, optional landlord field)
 
 **VocabularyMapping**:
-The translation from a Landlord's free-text description in a BulkUpload column (e.g. `"cavity: filledcavity"`) to a canonical domain enum value (e.g. `WallType.CAVITY`). Produced by a `ColumnClassifier` (today an LLM, tomorrow possibly a lookup table or rules engine) in the Model service. Stored per-Portfolio, one row per `(category, description)`. A row carries provenance (`classifier` or `user`) so user overrides survive re-classification.
+The translation from a free-text description to a canonical domain enum value (e.g. `"cavity: filledcavity"` → `WallType.CAVITY`). Two producers: the `ColumnClassifier` (an LLM in the Model service — needed because BulkUpload spreadsheets contain arbitrary landlord text that must be sanitised) and the **deterministic OS-code mapping** used by the postcode-search journey (a fixed OS classification code → enum table; no interpretation involved). Stored per-Portfolio, one row per `(category, description)`. A row carries provenance (`classifier`, `user`, or `os_places`) so user overrides survive re-classification and OS-derived facts stay distinguishable.
 _Avoid_: column mapping (that's a separate concept — see `ColumnMapping` above), classification, dictionary
 
+### Postcode search
+
+**Postcode search**:
+The journey that adds Properties to a Portfolio by searching a postcode: postcodes.io validates/normalises and supplies geography (local authority, constituency), OS Places supplies the addresses (UPRN, coordinates, classification code), and the user selects which addresses to add — across multiple postcode searches in one basket, submitted once. Raw OS responses are cached per-postcode (`postcode_search`) and re-served while fresh. Creates real Properties whose energy data is absent-until-modelled (derived state, no marker column); writes property type / built form facts through the Landlord-override layers via the deterministic OS-code mapping.
+_Avoid_: address import, quick add, single add
+
 ### Building parts
 
 **Building part**: