Add informal economy: counter-cyclical shadow economy with 4-channel tax evasion (#45)

[speedcom]

Summary

• Add counter-cyclical informal economy mechanism (INFORMAL_ENABLED toggle, default off)
• 4-channel tax evasion: CIT (firm-level), VAT, PIT, excise (aggregate-level)
• Per-sector shadow shares calibrated from Schneider 2023 / GUS (Agri 35%, Retail 30%, Health 20%, Mfg 15%, BPO 5%, Public 2%)
• Counter-cyclical dynamics: shadow share rises when unemployment exceeds threshold (lagged)
• 8 env vars, 4 new output columns (178-181), 3 new World fields, 31 new tests
• nCols 178 → 182; 1106 total tests passing; no new SFC identity (Identity 3 affected)

Test plan

• sbt test — 1106 tests pass (31 new in InformalEconomySpec)
• INFORMAL_ENABLED=true FIRM_SIZE_DIST=gus sbt "run 2000 3 informal_smoke pln" — EffectiveShadowShare ~0.57, TaxEvasionLoss ~32M PLN, no SFC FAIL
• INFORMAL_ENABLED=false sbt "run 2000 3 baseline_smoke pln" — all informal columns = 0, no SFC FAIL
• Gov debt higher with informality enabled (~33.5 vs ~32.4 mld PLN) confirming fiscal channel