Consolidate metric definitions with Rust JSON as single source of truth

- Add core.computed.json with 32 ratio definitions (filing + market derived)
- Add Rust types for ComputedDefinition and ComputationSpec
- Create generate-taxonomy.ts to generate TypeScript from Rust JSON
- Generate lib/generated/ (gitignored) with surfaces, computed, kpis
- Update financial-metrics.ts to use generated definitions
- Add build-time generation via 'bun run generate'
- Add taxonomy architecture documentation

Two-phase ratio computation:
- Filing-derived: margins, returns, per-share, growth (Rust computes)
- Market-derived: valuation ratios (TypeScript computes with price data)

All 32 ratios defined in core.computed.json:
- Margins: gross, operating, ebitda, net, fcf
- Returns: roa, roe, roic, roce
- Financial health: debt_to_equity, net_debt_to_ebitda, cash_to_debt, current_ratio
- Per-share: revenue, fcf, book_value
- Growth: yoy metrics + 3y/5y cagr
- Valuation: market_cap, ev, p/e, p/fcf, p/b, ev/sales, ev/ebitda, ev/fcf

.gitignore

@@ -13,6 +13,7 @@ build/
 dist/
 .next/
 out/
+lib/generated/
 
 # Environment
 .env
@@ -37,6 +38,9 @@ out/
 *.db
 *.sqlite
 
+# Generated code
+lib/generated/
+
 # Local app runtime state
 data/*.json
 data/*.sqlite