Rust refactor: derive Serialize/Deserialize on PendingStamp; drop hand-rolled JSON

[willwashburn]

Context

crates/relayburn-sdk/src/ingest/pending_stamps.rs:451-475 serialize_stamp and :406-444 parse_pending_stamp hand-roll JSON for PendingStamp. The struct already has Debug, Clone, PartialEq and the workspace already enables serde_json/preserve_order.

Adjacent: pending_stamps.rs:594-630 uuid_v4 shells out to read /dev/urandom per call. harnesses/claude.rs:62-105 mint_session_id rolls another v4-shaped UUID using DefaultHasher over now+pid — much weaker than OsRng, and the formatting code is 24 lines.

Proposed fix

1. Derive Serialize, Deserialize on PendingStamp with #[serde(rename_all = "camelCase")]. The two ad-hoc fns collapse to 1-line serde_json::to_string / from_str calls. ~80 LOC removed.
2. Replace both UUID generators with one consistent path. Two options:
   • Add uuid = "1" (pulls in ~one crate) with the v4 feature.
   • Add getrandom = "0.2" and keep a small format helper.
     The first is cleaner; the doc comments at harnesses/claude.rs explicitly avoided pulling uuid "for one call site," but now there are two.

Verification

• The *_compat_tests.rs files are explicitly named to catch byte-identical-ness with the TS sibling. Run cargo test -p relayburn-sdk pending_stamps after the change.
• The TS sibling produces JSON in a deterministic field order — verify the #[serde(rename_all = "camelCase")] field declaration order in the struct matches.

References

• crates/relayburn-sdk/src/ingest/pending_stamps.rs:406-444, 451-475, 594-630
• crates/relayburn-cli/src/harnesses/claude.rs:62-105
• Ingest+ledger and CLI review notes from the May 2026 Rust review.