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.

pysim_simple_server/server.py

@@ -7164,7 +7164,7 @@ def _test_command_type(name):
     return _TEST_COMMAND_TYPES.get((name or '').upper())
 
 
-_TEST_LOAD_MAX = 1000000
+_TEST_LOAD_MAX = 1000000000
 
 
 def _test_load_state(iterations, suite=False):