Fix:re-export XpFile, XpLayer, XpCell

[mors119]

##What
Re-exports XpFile, XpLayer, and XpCell from bracket-terminal::rex, restoring their availability through the existing bracket-lib and rltk facade chain.

##Why
rltk examples already rely on these RexPaint types being reachable through the facade path. Restoring the bracket-terminal::rex export point matches that existing structure while avoiding a broader bracket-rex prelude re-export that could introduce conflicts such as XpColor.

No related issue.

Checklist

Required

• cargo check --all passes
• cargo fmt --all -- --check passes
• cargo clippy --workspace --all-targets -- -D warnings -A clippy::multiple-crate-versions passes
• cargo test --all passes
• I linked the related issue (for example: Closes #123)

Functional Validation

• Behavior related to this change was verified locally (if applicable)
• Rendering/backend behavior was verified when runtime code changed (if applicable)
• Algorithm behavior (pathfinding/FOV/noise/random) was verified when affected (if applicable)
• I added or updated tests for changed behavior (if applicable)

Configuration & Docs

• User-facing docs were updated (README.md, ARCHITECTURE.md, or relevant manual pages, if applicable)
• New dependencies/configuration are documented (if applicable)
• No sensitive values or credentials were introduced

If Applicable

• Security impact considered (run cargo audit locally if needed)
• Breaking behavior changes are clearly described in this PR