fix(reactor): X_A_init default 0.001 → 200; add timeStep-unit regression test

Two fixes for the reactor unit-confusion drift surfaced in the 2026-05-19
wiki uplift:

1. X_A_init default in reactor.html was 0.001 g COD/m³, which is
   effectively zero nitrifying biomass — the reactor cannot nitrify
   ammonia under that initial condition (per the project memory note,
   ~50 mg/L is the minimum). Aligned to the schema default of 200 in
   generalFunctions/src/configs/reactor.json. Same change in
   test/helpers/factories.js so the test factory mirrors the operational
   default; tests that need low-biomass scenarios already override.

2. New test/basic/timestep-units.basic.test.js locks in the
   `config.timeStep is interpreted as seconds` contract — verifies the
   engine's days-stored / seconds-input invariant and asserts the
   schema declares `unit: "s"`, `default: 1`. Companion to the schema
   fix in the generalFunctions submodule.

Full test suite: 49/49 pass (was 46/46 + 3 new).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

test/helpers/factories.js

@@ -21,7 +21,7 @@ function makeUiConfig(overrides = {}) {
     X_S_init: 75,
     X_H_init: 30,
     X_STO_init: 0,
-    X_A_init: 0.001,
+    X_A_init: 200,
     X_TS_init: 125,
     timeStep: 1,
     enableLog: false,