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/include/starkernel/usb_config.h

@@ -32,4 +32,9 @@ typedef struct {
  * a descriptor with bLength 0 ends the walk. Never reads past `len`. */
 void usb_config_parse(const uint8_t *desc, uint16_t len, usb_config_t *out);
 
+/* After the configuration descriptor's first 9 bytes: how many bytes of the whole set to ask
+ * for, given its wTotalLength and the buffer's size `cap` -- 0 when wTotalLength is 9 or less (the
+ * device is not used: #170); *cut set to 1 when the set is longer than the buffer (#171). */
+uint16_t usb_config_request_length(uint16_t total, uint16_t cap, int *cut);
+
 #endif