feat: type permanent log discontinuities

[ZhiXiao-Lin]

Summary

• add stable cursor_lost and source_disconnected log discontinuity reasons
• expose exact unit, generation, cursor, and reason through RuntimeError
• document permanent discontinuities separately from retryable provider and transport failures

Validation

• cargo +1.85.0 fmt --all -- --check
• cargo +1.85.0 clippy --workspace --all-targets --locked -- -D warnings
• TMPDIR=/tmp cargo +1.85.0 test --workspace --all-targets --locked -- --test-threads=1
• RUSTDOCFLAGS=-D warnings cargo +1.85.0 doc --workspace --no-deps --locked