connect: preserve continuation on same-context refresh

[paintarm287]

Summary

Preserve the existing live next_tracks continuation when Spotify sends an UpdateContext for the same context and the current playback position can still be mapped successfully.

Previously, the current position was mapped correctly but next_tracks was then cleared and rebuilt. This could replace the established continuation with earlier tracks from the same playlist and replay tracks that had already been traversed.

Reproduction

Tested against dev at:

1599145bf2c98660d35b17817b3386767a7e4b42

With shuffle and repeat disabled:

• next_tracks decreased normally to 61.

• Spotify sent UpdateContext for the same 97-track playlist.

• next_tracks jumped from 61 to 79.

• The following skip decreased it to 78 and replayed a track already traversed earlier in the session.

Fix

For a same-context refresh where the current playback position maps successfully, retain the established live continuation instead of clearing and rebuilding next_tracks.

Genuine context changes continue to use the existing reset/rebuild path.

Validation

With the patch, same-context refreshes preserved the existing continuation:

• 77 -> 77

• 75 -> 75

• 74 -> 74

A subsequent manual skip test continued monotonically from 73 down through 45 with no upward queue reset and no unexpected replay.

The final qualification was also performed with the external replay-protection workaround disabled (SPOTIFY_REPLAY_GUARD=no), confirming that the corrected behaviour came from librespot itself.

Checks

• cargo fmt --all -- --check

• git diff --check

• live A/B reproduction against unmodified and patched dev

[paintarm287]

The failing Clippy check appears unrelated to this PR. CI is running Rust 1.98.0 and fails on clippy::needless_late_init in src/main.rs:1625, which is unchanged by this PR. The remaining CI checks, including builds and cross-compiles, pass.