load tests: raise the iteration cap to 1,000,000,000

Both layers: the server's _TEST_LOAD_MAX and the PWA's
TEST_ITERATIONS_MAX plus the two input max attributes.  The value stays
inside 2^53-1 (the JavaScript safe-integer range), so the counters and the
progress display survive the JSON/JS round trip exactly; the practical
ceiling is the run duration (50 ms/iteration ~1.6 years at the cap).

The server's range error names the new cap automatically.  The frontend
tests read the cap from index.html instead of stubbing a literal, and
clamp at it (above the cap, exactly at it, and the input max).

Docs: api.md, test-scripts.md, help EN/RU, the CHANGELOG (folded into the
unreleased 3.38.0) and the AGENTS notes.  sw cache simple-v479.
1061 Python + 1071 frontend.

CHANGELOG.md

@@ -22,6 +22,15 @@ every change carries a descriptive commit message.
   lists do not carry can still be typed, and the picker preselects the current
   path when it is known.
 
+### Changed
+
+- The load-test **iteration cap is raised to 1,000,000,000** (from 1,000,000) in
+  both layers: the PWA's `TEST_ITERATIONS_MAX` and the two field `max`
+  attributes, and the server's `_TEST_LOAD_MAX`.  The value is well inside the
+  JavaScript safe-integer range (2^53-1), so the counters and the progress
+  display stay exact; the practical ceiling is the run duration (at 50 ms per
+  iteration the cap is ~1.6 years).
+
 ## [3.37.0] — 2026-10-23
 
 ### Added