release: v0.3.19 structured transport errors

[JackCaow]

Summary

• align every Catcher npm, N-API, Rust, and Flutter package at version 0.3.19
• publish the structured N-API HTTP transport error contract merged in fix(http): preserve structured transport errors #28
• document stable error codes, retry causes, redaction behavior, and compatibility

Why this is manual

Release Please PR #17 proposed mixed 1.0.0 and 0.4.0 versions because it replayed historical commits. That violates this repository unified-version policy. This release PR follows the documented emergency release path and keeps all packages on one patch version.

Validation

• both Rust workspaces resolve at 0.3.19
• cargo check --workspace passed
• cargo clippy --workspace --all-targets -- -D warnings passed
• cargo test --workspace --no-fail-fast passed (local-proxy-only cases remain ignored by design)
• pnpm build:ts passed, including native N-API release builds
• pnpm typecheck passed
• git diff --check passed