fix: Arrow timestamps are in an _unknown_ timezone (#3504)

The current logic assumes a timezone-naive vortex.timestamp has the UTC
timzeone. However, Arrow [specifically says it is in an unknown
timezone](https://github.com/apache/arrow/blob/main/format/Schema.fbs#L304-L322):

> Timestamps with an unset / empty timezone
> -----------------------------------------
>
> If a Timestamp column has no timezone value, its epoch is
> 1970-01-01 00:00:00 (January 1st 1970, midnight) in an *unknown*
timezone.
>
> Therefore, timestamp values without a timezone cannot be meaningfully
> interpreted as physical points in time, but only as calendar / clock
> indications ("wall clock time") in an unspecified timezone.
>
>   For example, the timestamp value 0 with an empty timezone string
> corresponds to "January 1st 1970, 00h00" in an unknown timezone: there
>   is not enough information to interpret it as a well-defined physical
>   point in time.
>
> One consequence is that timestamp values without a timezone cannot
> be reliably compared or ordered, since they may have different points
of
> reference.  In particular, it is *not* possible to interpret an unset
> or empty timezone as the same as "UTC".

This bug most prominently appears in the Vortex cli browser which shows
the incorrect timezone for the min/max statistics of a timestamp column:

```
╭───────────────────────────────────────────────────────────────────
│ File Layout │ Segments
│╭Layout Info───────────────────────────────────────────────────────
││Kind: vortex.flat
││Row Count: 1
││Schema: {max=ext(vortex.timestamp, i64, ExtMetadata([1, 0, 0]))?,
││Children: 0
││Segment data size: 1.32 kB
││FlatBuffer Size: 1.32 kB
││
││
│╰──────────────────────────────────────────────────────────────────
│╭Array Info────────────────────────────────────────────────────────
││chunk                         max                           max_is
││0                             2025-06-11T02:24:37.054366Z   false
││
││
││
│╰──────────────────────────────────────────────────────────────────
╰───────────────────────────────────────────────────────────────────
```

After this change, the year, month, day, hour, etc. are unchanged, but
the timezone is correctly reported as unknown.

```
╭───────────────────────────────────────────────────────────────────
│ File Layout │ Segments
│╭Layout Info───────────────────────────────────────────────────────
││Kind: vortex.flat
││Row Count: 1
││Schema: {max=ext(vortex.timestamp, i64, ExtMetadata([1, 0, 0]))?,
││Children: 0
││Segment data size: 1.32 kB
││FlatBuffer Size: 1.32 kB
││
││
│╰──────────────────────────────────────────────────────────────────
│╭Array Info────────────────────────────────────────────────────────
││chunk                         max                          max_is
││0                             2025-06-11T02:24:37.054366   false
```

Signed-off-by: Daniel King <[email protected]>

Author: danking

vortex-dtype/src/datetime/temporal.rs

@@ -1,7 +1,7 @@
 use std::fmt::Display;
 use std::sync::{Arc, LazyLock};
 
-use jiff::civil::{Date, Time};
+use jiff::civil::{Date, DateTime, Time};
 use jiff::{Timestamp, Zoned};
 use vortex_error::{VortexError, VortexResult, vortex_bail, vortex_err, vortex_panic};
 
@@ -39,8 +39,8 @@ pub enum TemporalJiff {
     Time(Time),
     /// A date value.
     Date(Date),
-    /// A timestamp value.
-    Timestamp(Timestamp),
+    /// A zone-naive timestamp value.
+    Unzoned(DateTime),
     /// A zoned timestamp value.
     Zoned(Zoned),
 }
@@ -50,7 +50,7 @@ impl Display for TemporalJiff {
         match self {
             TemporalJiff::Time(t) => write!(f, "{t}"),
             TemporalJiff::Date(d) => write!(f, "{d}"),
-            TemporalJiff::Timestamp(ts) => write!(f, "{ts}"),
+            TemporalJiff::Unzoned(dt) => write!(f, "{dt}"),
             TemporalJiff::Zoned(z) => write!(f, "{z}"),
         }
     }
@@ -97,8 +97,8 @@ impl TemporalMetadata {
             TemporalMetadata::Timestamp(TimeUnit::D, _) => {
                 vortex_bail!("Invalid TimeUnit TimeUnit::D for TemporalMetadata::Timestamp")
             }
-            TemporalMetadata::Timestamp(unit, None) => Ok(TemporalJiff::Timestamp(
-                Timestamp::UNIX_EPOCH.checked_add(unit.to_jiff_span(v)?)?,
+            TemporalMetadata::Timestamp(unit, None) => Ok(TemporalJiff::Unzoned(
+                DateTime::new(1970, 1, 1, 0, 0, 0, 0)?.checked_add(unit.to_jiff_span(v)?)?,
             )),
             TemporalMetadata::Timestamp(unit, Some(tz)) => Ok(TemporalJiff::Zoned(
                 Timestamp::UNIX_EPOCH

vortex-scalar/src/display.rs

@@ -267,7 +267,7 @@ mod tests {
                     )))
                 )
             ),
-            "1970-01-04T02:05:10Z"
+            "1970-01-04T02:05:10"
         );
     }