Fix: is_sql=False deterministically implies 0% conversion

[shaypal5]

Problem

The simulation engine currently requires leads to pass through the SQL stage (via funnel progression mql → sal → sql → … → negotiation → closed_won) before they can convert. This means is_sql=False deterministically implies 0% conversion — a hard invariant that leaks outcome information into features.

This is documented as a known limitation in docs/v4/design.md:220 and tracked as a deferred engine fix in .agent-plan.md under "post-v1 list".

Root cause

In leadforge/simulation/engine.py, only leads at the negotiation stage are checked by ConversionHazard. Leads that never reach SQL can never reach negotiation, so they never convert.

Proposed fix

Add a rare "direct conversion" path: leads at pre-SQL stages (mql, sal) get a small daily probability of converting directly, bypassing the full funnel. This should be:

• Latent-score-modulated — higher latent → slightly higher direct conversion chance
• Much rarer than normal negotiation → closed_won path (e.g., 5% discount factor on the standard conversion hazard rate)
• Configurable — the discount factor should be a named constant, not a magic number
• The is_sql field semantics should be unchanged — it still reflects whether the lead reached SQL

Acceptance criteria

• With enough leads, some non-SQL leads convert
• Non-SQL conversion rate is much lower than SQL conversion rate
• Determinism is preserved (same seed → same result)
• All existing tests pass (with appropriate updates to tests that assumed is_sql=False → no conversion)

Labels

type: bugfix, layer: core, layer: simulation