refactor: merge crates/loro and crates/loro-internal into one crate

[zxch3n]

Summary

This merges crates/loro and crates/loro-internal into a single core crate.

• move the engine into crates/loro/src/internal/** and make loro the only core implementation crate
• preserve public LoroDoc auto-commit semantics while making the merged LoroDoc canonical
• keep the public Loro* / loro::event::* APIs as compatibility facades, and make loro::internal::* the canonical first-party low-level surface for container/value/event/diff/undo paths
• migrate crates/loro-wasm off loro-internal onto loro::internal without regressing the pending-event flush invariant
• delete crates/loro-internal, move compat tests/assets/version files into the merged layout, and update docs/tooling to the new structure

Notes

• The migration plan is tracked and marked complete in plans/20260306-merge-loro-crates.md.
• Compatibility strategy is intentionally conservative: public stable-facing APIs remain in place, while first-party hot paths use the canonical internal surface directly.
• Residual loro-internal text references are historical only (plan text, changelog text, and compatibility lockfiles for older pinned git revisions).

Validation

• cargo test -p loro
• cargo check --workspace
• cargo test -p loro-wasm
• pnpm -C crates/loro-wasm build-release
• cargo bench -p loro --bench merge_baseline -- --sample-size 10 --warm-up-time 0.1 --measurement-time 0.2

Benchmark Highlights

• internal heterogeneous reads: 1.4115 us .. 1.4424 us
• internal active subscriptions: 7.8933 us .. 8.1270 us
• internal undo callbacks: 9.0572 us .. 9.5479 us