feat(wassette): add stateful component mode

[JMLX42]

Summary

Implements opt-in stateful mode where Store/Instance persist per-component for the lifetime of the process, enabling in-memory state and WASI resource continuity across tool calls.

Closes #615

Implementation Tasks

• Create LoadOptions struct with stateful: bool field
• Create StatefulInstance struct for caching Store/Instance
• Add stateful_instances cache to LifecycleManager
• Update load_component to accept LoadOptions parameter
• Update execute_component_call for stateful mode
• Update unload_component to clean up stateful instances
• Write unit tests for stateful mode
• Update documentation

Design

See implementation plan for full design details.

Key characteristics:

• Opt-in: Enabled via stateful: true parameter at load time
• Per-component: Each stateful component gets one persistent Store/Instance pair
• Serialized execution: Concurrent calls to the same stateful component are queued
• Process-scoped: State lives until component unload or process exit

Test Plan

• Unit test: stateful component retains memory across calls
• Unit test: concurrent calls are serialized (not interleaved)
• Unit test: unload drops state, reload starts fresh

Note: Tests require fixing pre-existing compilation errors in mcp-server crate on upstream/main.

[JMLX42]

CI Failures - Pre-existing Issue on main

The CI failures are not caused by this PR. They are pre-existing on upstream/main due to a recent rmcp dependency update that added new required fields (meta, icons, title) to Tool and CallToolResult structs.

Verification:

$ git checkout upstream/main
$ cargo check --all
error[E0063]: missing field `meta` in initializer of `rmcp::model::CallToolResult`
# ... same errors

The wassette crate (which this PR modifies) compiles and tests pass:

$ cargo test -p wassette
test result: ok. 92 passed; 0 failed; 0 ignored

This PR should be mergeable once the mcp-server crate is updated to handle the new rmcp API.