fix(usb): a configuration of 9 bytes or less is refused, and a cut one is said (#170, #171)

After the configuration descriptor's first 9 bytes the event loop asked for
min(wTotalLength, 128) whatever it was. A wTotalLength of 9 or less was then
asked for again as "short" -- for ever, that slot never leaving enumeration
(#170); 0 also issued a zero-length data stage. A set longer than the 128-byte
buffer was cut without a word, its later interfaces never seen (#171).

The decision is usb_config_request_length() (moved out of the event loop,
then fixed): 9 or less is 0 -- "xhci: a configuration of N bytes has no
interface -- not used", and nothing more is asked; longer than the buffer is
cut and said -- "xhci: config of N bytes cut to 128 -- rest unseen" (at most
53 characters). The buffer stays 128 bytes; its size is #171's to rule.

kernel/tests/test_usb_config.c, seen to fail first against the decision as it
was (10 failures: wTotalLength 1 to 9 asked for again, the cut unsaid), now
20 checks, 0 failures. Both runs in logs/20261112-180140/kernel-tests/. Built for amd64,
aarch64 and riscv64; the boots come with the protocol. #171's other box, a
composite device on the SER5, is Captain Bob's.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017RzZgrf7V7Zk2iczBXDRbm

kernel/tests/test_usb_config.c

@@ -69,5 +69,18 @@ int main(void)
     usb_config_parse(set, 16, &c);
     CHECK(c.kind == USB_CONFIG_NO_INTERFACE, "an interface cut before its protocol byte: none found (kind %d)", c.kind);
 
+    /* #170: after the first 9 bytes, the set is asked for again in full -- a wTotalLength that is
+     * no longer than those 9 bytes would be asked for again, as "short", for ever */
+    {
+        int cut = -1;
+        uint16_t k;
+        for (k = 0; k <= 9; k++)
+            CHECK(usb_config_request_length(k, 128, &cut) == 0, "#170: wTotalLength %u: refused (0), not asked for again", k);
+        CHECK(usb_config_request_length(32, 128, &cut) == 32 && cut == 0, "a BOT stick's 32 bytes: asked for whole, not cut");
+        CHECK(usb_config_request_length(128, 128, &cut) == 128 && cut == 0, "exactly the buffer: whole, not cut");
+        /* #171: longer than the buffer -- cut, and said to be */
+        CHECK(usb_config_request_length(300, 128, &cut) == 128 && cut == 1, "#171: 300 bytes into 128: 128 asked for, and cut (%d) said", cut);
+    }
+
     return DONE();
 }