[olatechy0]

Summary

Adds a versioned JSON schema for decoded XDR values, replacing the previously unstable serde-shaped output.

• Every scval, envelope, and event document now carries schema_version: "1.0", kind, and value.
• Adds the global --raw flag (and DecodeOptions { include_raw: true }) to echo the canonical base64 input as raw_xdr. It is off by default.
• Documents the schema, XDR-JSON payload rules, versioning policy, and migration path in docs/XDR.md and docs/SCVAL.md.
• Adds golden snapshot tests for all three kinds plus structural schema validation tests and CLI coverage.
• Keeps existing kind/value fields, so the change is additive for consumers that ignore unknown fields.

Verification

Ran the project gate locally:

cargo fmt --all -- --check
cargo clippy --all-targets --all-features -- -D warnings
cargo test --all

Manual check:

$ stellar-foundry --format json scval AAAAAwAAACo=
{"kind":"scval","schema_version":"1.0","value":{"u32":42}}
$ stellar-foundry --format json --raw scval AAAAAwAAACo=
{"kind":"scval","raw_xdr":"AAAAAwAAACo=","schema_version":"1.0","value":{"u32":42}}

Closes #12