Centralize the medical code-system OIDs/URIs (LOINC/SNOMED) into one source

[nickclyde]

Problem

The LOINC OID 2.16.840.1.113883.6.1 (and SNOMED/LOINC URIs) are hardcoded in multiple places that must stay in sync:

• packages/augmentation/.../eicr_augmenter.py:283 (translation codeSystem)
• packages/text-to-code-lambda/.../lambda_function.py:397 (new Code.code_system)
• packages/text-to-code/.../models/evaluator.py — CodeSystemValues plus duplicated inline literals across LabTestNameOrdered*/LabTestNameResulted* criteria.

Proposed scope

• Add a small constants module (e.g. shared_models.code_systems with LOINC_OID, LOINC_URI, SNOMED_OID, SNOMED_URI, names) since both augmentation and text-to-code already depend on shared-models.
• Replace the hardcoded literals at the sites above with the shared constants; have evaluator.CodeSystemValues reference them too.

Acceptance criteria

• No raw LOINC/SNOMED OID string literals remain in the listed files; rg '113883\.6\.1' across packages/ resolves to the single constants module. Existing tests still pass.