C2: Failure days drawn per exposure event; worst-case loses the positive LRVs of failing steps #25

Status: review (built in pull request #29, CI green on a8a1972; stays open until the release, D5)

Source

Spec C2 Failure calculation (docs/specs/c2-failure-calculation.md), ADR-0001, roadmap decision D4. The Spec issue does not exist yet.

What to build

The worst-case result includes treatment failures. For every simulated exposure event, and for every treatment step with a failure frequency above 0, the step fails that day with probability failure frequency / 365 (ADR-0001). On a failure day, worst-case calculates the event with the positive minimum LRVs of every failing step removed, whatever the failure duration. LRVs of 0, below 0 or empty stay in place (D4).

The failure draws come from their own fixed-seed random generator and are shared by both cases and all reference pathogens. Raising a failure frequency therefore only adds failure days. If every failure frequency is 0, no failure draws are made and the results are exactly today's. Best-case does not change in this Ticket; the mixed-water assumption follows in the next one.

The result page, the reference-level exceedance, saved results and the export's result table show whatever the calculation returns. Their layout does not change.

Acceptance criteria

• With every failure frequency at 0, all results equal today's exactly, including the existing regression test's numbers.
• A step with failure frequency 365 gives the same worst-case results as the same scenario without that step's positive minimum LRVs.
• Raising a failure frequency never lowers any mean. Changing a failure duration leaves worst-case unchanged.
• Worst-case mean ≥ best-case mean for every reference pathogen and risk measure, with and without failures.
• D4: a failing step with a negative LRV in one pathogen group and a positive LRV in another leaves the first group's results unchanged. A group with LRV 0 is unaffected.
• Calculating the same scenario with failures twice gives identical results.
• All tests go through the calculation's public entry point and check only the returned results. The existing test suite passes unchanged.

Blocked by

• C2: Tidy-up: the calculation works per exposure event, with results unchanged #24
• C1: Failure inputs on a treatment step in the configurator (enter, check ranges, save, reopen, edit) #21 (C1: the stored failure frequency and failure duration)