test: deterministic simulation campaigns over a simulated network, and the fixes they found #34

zig build pbt -Dhegel runs every campaign; zig build pbt-<name> -Dhegel runs one. Ordinary builds and zig build test do not need Hegel.

• xrpc: wire encoding, bearer auth, and a retry model (its own backoff schedule, not the policy's) layered over zttp's dead-connection replay, with Retry-After and reset hints, refused, reset, and truncated connections, and keep-alive reuse.
• identity: PLC and did:web resolution, the DoH preflight against 19 hand-classified answers (IPv4 and IPv6, routable, special-purpose per the IANA registries, and non-literal) judged alone and in mixes, a preflight pass that must reach TLS setup, and handle TXT did= selection.
• firehose: FirehoseClient over simulated relays and websocket.zig: the cursor on every reconnect, the exact delivered sequence, backoff with its reset and cap, host rotation, and the Host header. LabelClient shares this code.
• oauth: dpopRequest and refreshAccessToken: nonce challenges in body and header, missing nonces, exhaustion, each proof's claims, the returned nonce, and leaks under a checking allocator.
• mst: one-shot block reader failures during lazy reads, checked for atomicity.
• codecs, runtime: the earlier codec, syntax, crypto, and virtual-clock properties.

zig build mutants -Dhegel plants the 20 entries of pbt/mutants.zon, including the inverse of every fix above, and counts the seeds that catch each. Two new oracles were checked against the harness before them: the block-reader mutant is caught on 0 of 10 seeds before reader faults and 10 of 10 after; a backoff mutant on 0 of 10 when the XRPC model called delayMillisAt and 10 of 10 with its own schedule.

pbt/findings.md records each finding with its replay blob.

• zttp returns a body cut short of Content-Length as complete. Fixed on zttp fix/content-length-truncation; pbt/xrpc.zig counts the divergence behind transport_reports_truncation until zat pins a release with it.
• websocket.zig drops a handshake header whose CRLF splits across reads. Fixed on websocket.zig fix/handshake-split-crlf; the simulator avoids that split behind split_upgrade_crlf and counts it.
• open: zttp's dead-connection replay resends a single-use DPoP proof (RFC 9449 §11.1). Fixing it needs a per-request replay opt-out in zttp or a zat retry with a fresh proof.

The simulator runs async inline and has no concurrent, so zttp's deadline and stall guards and idle_timeout_ms are untested. TLS is not simulated: wss, https well-known handle lookups, did:web document fetches, and JetstreamClient, which always connects over TLS, are not covered.

Each of the 24 commits passes zig build check, zig build test, and, where campaigns exist, a seeded zig build pbt -Dhegel. The tip compiles the tests for arm-linux-musleabihf. The JWT isExpiredAt and RFC 6979 commits are their own PR (33, cell-extras) and are not part of this one.