chore: fix pre-existing clippy warnings (dead code + unused imports)

[ax1s-x1zz]

Summary

The compiler crate builds with 3 pre-existing lint warnings. Cleaning them up makes cargo clippy --workspace --all-targets output clean for contributors and is a good first introduction to the codebase.

Findings

1. xazz-compiler/src/checker.rs:1142 — fn column_missing is never used (dead_code). It calls column_missing_with_available(col, ctx, &HashMap::new()); callers were migrated to the richer helper. Delete the dead method and verify no call sites remain.
2. xazz-core/src/error.rs:291 — use crate::i18n::{is_korean, tr}; is unused (unused_imports). Confirm nothing in the file uses them (the suggestion/display code was refactored), then remove the import.
3. xazz-exec/src/chart.rs:361 — in #[cfg(test)] mod tests, use xazz_compiler::ast::{ChartConfig, ChartType}; is unused. Remove the import (or use it if a test intends to reference those types).

Definition of Done

• cargo clippy --workspace --all-targets produces zero warnings for these three items (pre-existing column_missing warning gone; unused imports gone).
• cargo test --workspace still passes (full suite, expect ~298 tests; xazz-exec builds can be slow).
• cargo fmt --all -- --check is clean.

Notes

• Do not run cargo fix --allow-dirty blindly; only touch the three items listed.
• The repository convention is English-first user output; if you touch user-facing strings, keep the existing i18n (tr/is_korean) pattern.