fix: avoid fork_at panics

[zxch3n]

Summary

• make Rust fork_at return LoroResult and propagate errors through wasm forkAt
• replace panic/unwrap/unimplemented points on the fork_at snapshot export/import path with typed errors
• add regressions for invalid frontiers and shallow docs, plus a wasm-side throw case

Testing

• cargo test -p loro fork_at
• cargo test -p loro test_iter_change_on_edge -- --exact
• cargo test -p loro-wasm --lib fork_at -- --exact