perf: improve performance of timestamp truncate

[andygrove]

Which issue does this PR close?

Closes #2995

Rationale for this change

Sub-day truncations (MICROSECOND, MILLISECOND, SECOND, MINUTE, HOUR) are timezone-independent - truncating to the minute boundary is the same regardless of timezone display. This allows simple integer arithmetic instead of expensive DateTime conversions.

Sub-day Formats (Optimized with Arithmetic)

Format	Before	After	Speedup
HOUR	957 µs	32 µs	~30x faster (96.6% improvement)
MINUTE	640 µs	32 µs	~20x faster (95% improvement)
SECOND	405 µs	32 µs	~12x faster (92% improvement)
MILLISECOND	430 µs	32 µs	~13x faster (92.6% improvement)
MICROSECOND	435 µs	27 µs	~16x faster (93.7% improvement)

Non-UTC Timezone (America/New_York)

Format	Before	After	Speedup
SECOND	598 µs	35 µs	~17x faster (94.2% improvement)

What changes are included in this PR?

How are these changes tested?

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.58%. Comparing base (f09f8af) to head (75a53cb).
⚠️ Report is 803 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #2996      +/-   ##
============================================
+ Coverage     56.12%   59.58%   +3.46%     
- Complexity      976     1377     +401     
============================================
  Files           119      167      +48     
  Lines         11743    15493    +3750     
  Branches       2251     2569     +318     
============================================
+ Hits           6591     9232    +2641     
- Misses         4012     4962     +950     
- Partials       1140     1299     +159     

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