fix: validate StickyIndex sequence owners

[JoongWonSeo]

Summary

• add StickyIndex.resolve(sequence) -> int | None with exact resolved Yrs branch validation for Text and Array
• make sequence-backed get_index() reject sibling, deleted, detached, or unavailable owners instead of returning a foreign numeric offset
• replace StickyIndex decode/resolution unwrap() and unsafe conversion paths with Python ValueError/None outcomes
• support valid empty/end positions and reject out-of-range construction
• preserve serialized formats and transaction-based get_index() compatibility

Coverage includes both associations, Text/Array and cross-kind mismatches, start/interior/end and empty positions, cross-replica and fresh-wrapper resolution, sibling/deleted/detached/unavailable owners, malformed input, and legacy behavior. It also locks in nested Map-owned end-sentinel behavior across binary decode and replica transfer, while distinguishing a relative index-zero anchor and retaining wrong-owner rejection.

Validation

• cargo check --all-targets
• cargo test --all-targets — 6 passed
• Ruff check and format
• mypy
• Python suite with coverage — 289 passed, 4,568/4,568 statements
• MkDocs build

Closes #422.