fix: correct DCF engine — FCFF/WACC consistency, smooth growth fade, mid-year convention

[blackswan127]

Summary

Six correctness fixes to the DCF valuation engine, covering financial accuracy, numerical stability, and code quality.

Changes

1. FCFF / WACC consistency

• dcfService.ts: computes historicalFCFF server-side by adding back after-tax interest expense to reported (levered) FCF
• Client DCF model now uses unlevered FCFF-derived margins, consistent with the WACC denominator that includes both equity and debt capital
• Quarterly TTM rows correctly carry zero interest add-back when no interest data is present

2. SEC EDGAR confidence period fix

• dcfService.ts: captures latestAnnualFCF / latestAnnualRev before TTM is appended
• dataConfidence block now compares annual-vs-annual data, eliminating false-medium signals caused by the TTM period mismatch

3. Smooth 2-stage growth fade for any projection length

• dcfCalculator.ts: replaces the hard-coded 5-year stage-1 cliff with adaptive split — stage1Years = min(5, ceil(N/2))
• For a 6-year projection: stage-1 = 3 years, smooth linear fade over years 4–6 (no cliff)
• For a 10-year projection: stage-1 = 5 years, fade over years 6–10

4. Monte Carlo initial-margin standard deviation

• monteCarlo.ts: separates fcfMarginStdDev (initial-year dispersion) from targetFcfMarginStdDev (long-run target dispersion)
• Fixes silent conflation that was underestimating initial margin uncertainty

5. Deduplicated projection core

• dcfCalculator.ts: extracts runProjection(inputs, wacc, currentPrice) as a single shared function
• computeDCF and computeDCFWithFixedWACC both delegate to it, eliminating ~80 lines of duplicated logic

6. Mid-year discounting convention

• dcfCalculator.ts: adds optional midYearConvention?: boolean flag
• When enabled: year-i FCF discounted at period i − 0.5; terminal value at period N − 0.5
• Off by default to preserve all existing test values
• UI: checkbox in the WACC card; wired into all four input objects and Monte Carlo

Tests

• 4 new tests added: 6-year smooth fade, mid-year discounting accuracy, shared-core consistency (computeDCFWithFixedWACC matches computeDCF at the same WACC), FCFF series assertions on the DCF route
• All 49 tests pass (vitest run)
• npx tsc --noEmit exits 0
• Production build succeeds (vite build)

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