sp-io: report runtime panic messages to the host by default

[hitchho]

Description

Makes improved_panic_error_reporting the default behavior of sp-io's runtime panic handler: panics (and OOM aborts) now marshal their message to the host via the PanicHandler::abort_on_panic host function instead of logging and executing unreachable. Callers get Runtime panicked: <message> (sc_executor_common::Error::AbortedDueToPanic) instead of the opaque:

Execution failed: Execution aborted due to trap: wasm trap: wasm `unreachable` instruction executed

The old behavior remains available behind a new opt-out feature, legacy_panic_error_reporting, for runtimes that must target a host without the PanicHandler host functions. improved_panic_error_reporting is kept as a deprecated no-op so existing Cargo.tomls don't break.

Motivation

The abort_on_panic mechanism has existed for years, yet outside sc-executor's own test runtime it appears to have virtually no production adoption — chains ship with the opaque-trap default and users pay for it at debugging time. Concrete example from galacticcouncil/hydration-node#1491: a SCALE decode failure of a runtime-API parameter (Bad input data provided to dry_run_call: Codec error…) surfaced over public RPC as a bare wasm trap, was misdiagnosed as a runtime execution bug, and took a chopsticks fork to root-cause — the actual panic message only ever reached the node's local logs. With this change the message reaches the RPC caller directly.

Compatibility

• sc-executor has captured the panic message (HostState::panic_message → AbortedDueToPanic) since 2022.
• smoldot implements ext_panic_handler_abort_on_panic_version_1.
• Polkadot PVF validation: the PVF executor's host function list does not include PanicHandler, but it instantiates with allow_missing_func_imports: true, so PVFs built with the new default still prepare and execute; a panic during validation traps just as it does today (outcome-identical: candidate invalid).
• Hosts predating the host function (or custom embedders without it) can build the runtime with legacy_panic_error_reporting.

Review notes

• substrate/client/executor/runtime-test already enables improved_panic_error_reporting, so executor tests exercise the new-default path today; trap-asserting tests (call_not_existing_function, unreachable_intrinsic, host-allocator failure) cover genuine traps and are unaffected.
• The deprecated no-op feature can be removed in a follow-up major release.