http: refactor scry handling, plug a leak

[dozreg-toplud]

On develop, if a malformed scry URL is queried, we get a leak that ASAN can detect. For example, after hitting F5 ten times at http://localhost:8080/_~_/= and then pressing ctrl-D:

Direct leak of 240 byte(s) in 10 object(s) allocated from:
    #0 0x798dcccf6cbf in malloc (/usr/lib/clang/18/lib/linux/libclang_rt.asan-x86_64.so+0xf6cbf) (BuildId: 4cd39e6608b20f2f5a148a941cd434e0cadcd3dc)
    #1 0x1cd2706 in _http_req_dispatch /home/kirill/work/urbit-repos/vere/pkg/vere/io/http.c:983:29
    #2 0x1cd0dc6 in _http_rec_accept /home/kirill/work/urbit-repos/vere/pkg/vere/io/http.c:1676:7
    #3 0x1ce0378 in call_handlers /home/kirill/.cache/zig/p/N-V-__8AAJ2XqALM9jST2Gu4TIdjPNY2QB8rnmKeepeqr076/lib/core/request.c:155:13
    #4 0x1ea55ca in handle_incoming_request /home/kirill/.cache/zig/p/N-V-__8AAJ2XqALM9jST2Gu4TIdjPNY2QB8rnmKeepeqr076/lib/http1.c:470:13
    #5 0x1bdd68e in uv__read /home/kirill/.cache/zig/p/N-V-__8AAH34QwB6wi5eQK_lFbfDGSN3hRE8l-6Ep198ZsGg/src/unix/stream.c:1148:7
    #6 0x1bdb3b7 in uv__stream_io /home/kirill/.cache/zig/p/N-V-__8AAH34QwB6wi5eQK_lFbfDGSN3hRE8l-6Ep198ZsGg/src/unix/stream.c:1208:5
    #7 0x1bcfc3c in uv__io_poll /home/kirill/.cache/zig/p/N-V-__8AAH34QwB6wi5eQK_lFbfDGSN3hRE8l-6Ep198ZsGg/src/unix/linux.c:1565:11
    #8 0x1bca927 in uv_run /home/kirill/.cache/zig/p/N-V-__8AAH34QwB6wi5eQK_lFbfDGSN3hRE8l-6Ep198ZsGg/src/unix/core.c:460:5
    #9 0x1bbe425 in u3_king_commence /home/kirill/work/urbit-repos/vere/pkg/vere/king.c:1061:3
    #10 0x1323f4e in main /home/kirill/work/urbit-repos/vere/pkg/vere/main.c:3320:5
    #11 0x798dcc829d8f in __libc_start_call_main csu/../sysdeps/nptl/libc_start_call_main.h:58:16

This PR factors out scry-over-HTTP handling into a separate function _http_peek_dispatch with semitransfer semantics for req_u->peq_u and the contained noun.