Cap a report at 32 KB on both sides of the upload

The client would send up to 256 KB and the sink answered a settling 400 above
64 KB, so the two disagreed about what a valid report is, and the client
deletes what it is told is invalid. Unreachable in practice — the module table
and backtrace are both bounded, which puts the ceiling near 10 KB — but the
two constants have to agree for the disagreement not to matter.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

crash-telemetry/test.mjs

@@ -43,7 +43,7 @@ assert.equal(await sent.body.get("files[0]").text(), REPORT);
 
 // junk: client should delete these, so they must be 400
 assert.equal((await post("hello")).status, 400);
-assert.equal((await post("x".repeat(64 * 1024 + 1))).status, 400);
+assert.equal((await post("x".repeat(32 * 1024 + 1))).status, 400);
 
 // throttled: 429, and nothing reaches Discord. reportIsSettled() leaves 429
 // unsettled, so the client keeps the report for next launch.