fix: warn when native and JVM timezone databases differ

[andygrove]

Which issue does this PR close?

Closes #6331.

Rationale for this change

Spark converts between instants and local time using the JVM's timezone rules (tzdb.dat). The version of those rules depends on the JDK build and on whether TZUpdater has been run. Comet's native kernels use chrono-tz instead, which compiles its own copy of the IANA database into libcomet: chrono-tz 0.10.4 carries 2025b.

When the two versions disagree about a zone, every local-time operation on the affected timestamps returns a different answer, and nothing reports it. For example, with AppleJDK 17.0.10, which ships tzdata 2023c, hour(ts) for 2025-06-15T12:00:00Z in America/Asuncion is 8 in Spark and 9 in Comet.

Matching Spark in every deployment would mean making native code use the JVM's rules, which is a much larger change. This PR makes the mismatch visible instead.

What changes are included in this PR?

• A JNI function, NativeBase.getTzdataVersion, returns the database version compiled into libcomet (chrono_tz::IANA_TZDB_VERSION). chrono-tz becomes a direct dependency of datafusion-comet. It was already in the dependency tree through arrow's chrono-tz feature, so the only Cargo.lock change is that one dependency line.
• When the native library loads, NativeBase compares that version with the JVM's (ZoneRulesProvider.getVersions("UTC").lastKey()) and logs a warning if they differ. The warning names both versions.
• A new "Timezone Database Versions" section in the datetime compatibility guide explains the difference and which operations it affects.
• A test in CometTemporalExpressionSuite checks that the version is reported and that the warning is built.

How are these changes tested?

• The new test and all 36 tests in CometTemporalExpressionSuite pass on Spark 4.1, including the spotless and scalastyle checks.
• On this machine the suite's log shows the warning with the real versions: Comet's native library uses timezone database 2025b, but the JVM uses 2023c.
• cargo clippy --all-targets -- -D warnings is clean for datafusion-comet.
• prettier --check passes on the edited docs page.