Declare the serialization-dependent example feature

At clean Zheng c753f5ae055d3f4a43847877521f119d3b40dd39, the full default workspace test command failed with TraceProof: serde::Serialize at rs/examples/legacy_wire_cost.rs:29. That example serializes its proof, while TraceProof serialization is gated by serde. Its target lacked the matching Cargo feature requirement.

The repair adds only required-features = ["serde"] to an explicit legacy_wire_cost example declaration. Cargo.lock and dependency versions are unchanged. Default builds select supported targets; enabling serde still compiles the serialization example.

receipt.json binds the original source identities, exact minimal diff, commands, dependency revisions, clean statuses, and lossless compressed logs. The repaired base was tested in a separate real worktree with only rs/Cargo.toml modified:

• cargo check --workspace --all-targets --all-features --release --locked --offline passed with zero warnings and includes the serde-enabled example.
• cargo test --workspace --release --locked --offline -- --test-threads=4 passed: 215 tests, zero failures, one existing ignored test, zero warnings.

Both commands use the explicit CARGO_TARGET_DIR and working directory in the receipt. The failed original command/log remains retained. These measurements precede the fix commit and refer to base c753 plus the recorded manifest diff.