[Search] Implement alias expansion layer from data/aliases/common.json

Problem summary

Alias expansion is documented in README and MatchMethod includes 'alias', but packages/search/src/layered.ts does not load or apply data/aliases/common.json. Queries like DM, HTN, and T2DM do not expand to canonical terms.

Affected area

• data/aliases/common.json
• packages/search/src/layered.ts
• app/components/CodeCard.tsx (alias badge label exists but is never shown)

Expected behavior

Common clinical abbreviations expand before layered search runs. Expanded hits surface with matchMethod: 'alias'.

Actual behavior

Only exact → prefix → substring → fuzzy layers run. Alias file is unused.

Reproduction

1. Open ATC-5 or ICD-10.
2. Search DM or HTN.
3. Compare with searching diabetes mellitus or hypertension.

Acceptance criteria

• Alias table is loaded at search index build time.
• Layered search applies alias expansion before or alongside other layers.
• Results from alias expansion use matchMethod: 'alias'.
• Unit tests cover at least 5 common abbreviations from common.json.
• README architecture diagram matches implementation.

Start with data/aliases/common.json and packages/search/src/layered.ts to trace how the search index is built and how layers assign matchMethod values. Check app/components/CodeCard.tsx and the README architecture diagram for the alias result presentation. Add coverage for at least five abbreviations, verify alias-expanded results use matchMethod 'alias', and confirm the documented architecture matches the implementation.