feat(wasm-runtime): complete host function infrastructure for WASM challenges

[echobt]

Summary

Closes multiple gaps in the WASM runtime infrastructure so that external challenge modules (e.g. term-challenge) built against platform-challenge-sdk-wasm can be loaded with full feature support — container execution, challenge data loading, correct ABI handling, and proper storage wiring.

Changes

Storage host functions

• Register storage_delete and storage_propose_write handlers in StorageHostFunctions::register() (previously declared but never linked)
• Add guest-side extern "C" declarations and wrapper functions in the SDK

ABI & type alignment

• Fix get_tasks executor to use call_return_i64 and unpack packed ptr|len (was incorrectly calling call_return_i32 + non-existent get_tasks_result_len)
• Add missing metrics and details fields to executor-side EvaluationOutput (with #[serde(default)] for backward compat)
• Add task_definition and environment_config to executor-side EvaluationInput
• Fix bridge score conversion: divide by 10_000.0 instead of 100.0 to match score_f64_scaled

Container execution delegation (platform_container)

• New container.rs module with container_run host function, ContainerPolicy, ContainerRunRequest/ContainerRunResponse types
• Executes containers via docker run with configurable image allowlist, memory/CPU limits, network mode, timeout, and per-execution container count cap
• Registered in runtime.rs linker; guest-side types exported from SDK

Challenge data loading (platform_data)

• New data.rs module with data_get and data_list host functions, DataPolicy, filesystem-backed DataBackend
• Allows WASM modules to read challenge-specific data files (task definitions, configs) by key with path traversal protection
• Registered in runtime.rs linker

Storage backend wiring

• WasmExecutorConfig now accepts an Arc<dyn StorageBackend> and passes it through to InstanceConfig (replaces NoopStorageBackend)
• InstanceConfig fields for storage, container, and data state are set explicitly instead of relying on defaults

Arena allocator

• Add huge-arena feature (16 MiB) to challenge-sdk-wasm for complex challenges with large payloads
• Properly gate large-arena vs huge-arena vs default with cfg precedence

Host function signature fix

• Align http_get host registration signature with guest SDK declaration (5 params)

Testing

• All existing tests pass (cargo test --workspace)
• New unit tests for container policy, status codes, request serialization, and data backend

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch feat/wasm-runtime-infrastructure-gaps-c755vt

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