Add endogenous firm entry

[speedcom]

Summary

• Bankrupt firm slots are recycled as new micro entrants with profit-weighted sector choice, sector-specific entry barriers (GUS CEIDG/KRS 2024), and optional AI-native startups
• 7 env vars (FIRM_ENTRY_ENABLED, FIRM_ENTRY_RATE, FIRM_ENTRY_PROFIT_SENS, FIRM_ENTRY_SECTOR_BARRIERS, FIRM_ENTRY_AI_THRESHOLD, FIRM_ENTRY_AI_PROB, FIRM_ENTRY_STARTUP_CASH), toggled off by default
• 4 new output columns (171-174): FirmBirths, FirmDeaths, NetEntry, LivingFirmCount; nCols 171 → 175

Test plan

• 28 new tests in FirmEntrySpec (config defaults, entrant properties, AI-native/traditional, sector choice, profit signals, entry probability)
• IntegrationSpec + IntegrationFullSpec updated for nCols=175
• Full test suite: 1042 passed, 0 failed
• Smoke run: FIRM_ENTRY_ENABLED=true FIRM_SIZE_DIST=gus sbt "run 2000 3 entry pln" — verify FirmBirths > 0, no SFC FAIL