Move temporal cast helpers into a private submodule

[anandghegde]

Which issue does this PR close?

Part of #11032 — this is the temporal helpers group only, so it does not
close the issue. The remaining groups (numeric/boolean, binary/byte-view,
struct, decimal adapters) are untouched here. @amitvijapur mentioned taking the
struct group; this branch does not touch it.

Rationale for this change

arrow-cast/src/cast/mod.rs is ~14k lines. #5125 moved list, decimal,
dictionary, string, map, run-array and union support into private submodules,
and #11032 captures the groups still left inline. The timestamp, date,
duration, interval and timezone helpers are one such cohesive group.

What changes are included in this PR?

A new private arrow-cast/src/cast/temporal.rs holding:

• cast_interval_year_month_to_interval_month_day_nano
• cast_interval_day_time_to_interval_month_day_nano
• cast_month_day_nano_to_duration
• cast_duration_to_interval
• make_timestamp_array, make_duration_array
• as_time_res_with_timezone
• timestamp_to_date32
• adjust_timestamp_to_timezone

cast_with_options stays the top-level dispatcher in mod.rs.

Two things worth flagging for review:

• cast_reinterpret_arrays deliberately stays in mod.rs. It sits in the
  middle of this block, so it looks like part of the group, but it is a generic
  PrimitiveArray::reinterpret_cast wrapper with 23 call sites across the
  numeric, temporal and dictionary paths — not temporal-specific.
• The helpers became pub(crate), which is what they need to stay reachable
  from mod.rs through the glob import, matching string.rs and list.rs.
  Nothing new is exposed outside the crate.

The bodies are moved verbatim. I checked this mechanically rather than by eye:
extracting the three original line ranges from HEAD and normalising away the
pub(crate) prefix gives a byte-identical match against the new file.

Are these changes tested?

Covered by the existing arrow-cast tests — this is a code move, so no new
tests are warranted and no test was modified.

cargo test -p arrow-cast     379 passed + 12 passed, 0 failed (same as base)
cargo clippy -p arrow-cast --all-targets -- -D warnings   clean
cargo fmt -p arrow-cast -- --check                        clean
cargo build -p arrow                                      builds

Run on macOS 26.6.2, aarch64-apple-darwin, toolchain 1.98.1 from
rust-toolchain.toml.

Are there any user-facing changes?

No. No public API change, no behavioural change. Everything moved is private to
the crate, and the two mod.rs imports dropped (IntervalMonthDayNano, and
NaiveTime/Offset from chrono) were orphaned by the move.

AI usage disclosure

Written with AI assistance (Claude). The AI performed the mechanical extraction
and ran the verification above; I reviewed the result and own the change.

To be concrete about what that means here, since the value of a code move is
entirely in it being faithful: the moved bodies are unedited, and the
byte-identity check described above is the evidence for that rather than a
reading. The two judgement calls — leaving cast_reinterpret_arrays behind, and
the pub(crate) visibility — are called out above precisely because they are the
parts a diff of a "pure move" would not otherwise draw attention to.