Cut over public capability ABI to Synapse

[lthibault]

Summary

• introduce Synapse as the public capability ABI and wire it through membrane exports, process bootstrap, vat serve/dial, stdio serve, graft, and HTTP/stream forwarded caps
• add descriptor validation and Synapse helper substrate while preserving Glia perform interception before backend capability invocation
• remove legacy schema sidecar machinery and the incomplete auction / mindshare examples

Notes

• This is the planned breaking ABI cutover before recursive attenuation; there is no compatibility shim.
• The Synapse substrate is intentionally minimal. Recursive attenuation policy, typed facades/codegen, and a public custom Invokable server API remain out of scope.

Tests

• cargo fmt --check
• cargo check --workspace --tests
• RUSTC=$(rustup which rustc) make -C examples/echo
• cargo test --workspace