fix: anchor text cursors at non-BMP UTF-16 boundaries - #1103

Fixes #1101.

LoroText.getCursor() can return undefined at the start of an emoji when that character occupies its own text atom. It can also accept a position inside a surrogate pair and anchor it at the preceding boundary.

The stable-position lookup now verifies that the queried cursor maps back to the requested index and, for WASM event indices, selects the full UTF-16 width of the character. Native Unicode indexing, cursor sides, and end-of-document behavior are preserved. No public signatures or serialized formats change.

Regression coverage includes mixed non-BMP text, combining sequences, mid-surrogate positions, separate-peer atoms, cursor resolution after remote edits, and native Rust controls. A patch changeset is included; the code comment explains why the index round trip is necessary.

Validation

• Demonstrated the WASM regression failing on the base build and passing with the fix.
• Release WASM build and package tests passed: 375 Vitest tests, TypeScript, CommonJS smoke, 4 Deno tests, and 4 Bun tests.
• Native cursor contract selection: 9 passed; internal rich-text selection: 60 passed; cargo check -p loro-internal passed.
• Independent review reran all 27 tests in the changed WASM rich-text file and the 9 native cursor tests, plus additional probes at formatting boundaries.
• Changed Rust files pass rustfmt; git diff --check passes.

Repository-wide formatting and Clippy are not green on the unmodified base with the local Rust 1.98 toolchain (unrelated formatting differences and existing generic-btree diagnostics). No unrelated formatting or dependency changes are included. WASM package tests used a temporary empty PostCSS configuration to avoid an unrelated configuration in the parent directory; that workaround is not part of this patch.