…riter (apache#62544)

When writing nullable columns, `append_nullable` splits each block at
null boundaries into small non-null runs.

| Input block | add_values calls |

|--------------------------------------|-----------------------------------------------------------|
| `[1, 3, NULL, NULL, 4, 5, 6, NULL]` | `add_values([1,3], count=2)` +
`add_values([4,5,6], count=3)` |

Each `add_values` call built two heap-allocated `Field` temporaries just
to compare
min/max — allocation, memcpy, variant dispatch, destructor — per call.

Track page min/max as raw `ValType` (the same native type
`std::minmax_element` already uses). Materialize `Field` only once at
`flush()` time.

`block_rows` = post-null-split run length.

| Type | block | Baseline (M items/s) | Optimized (M items/s) | Speedup
|
|---|---:|---:|---:|---:|
| Int32 | 1 | 21.9 | 240.3 | **11.0×** |
| Int32 | 4 | 75.9 | 353.0 | **4.7×** |
| Int32 | 16 | 170.0 | 400.1 | **2.4×** |
| Int32 | 64 | 261.9 | 394.4 | **1.5×** |
| Int32 | 256 | 314.0 | 419.7 | 1.3× |
| Int32 | 1024 | 320.6 | 425.9 | 1.3× |
| Int32 | 4096 | 334.2 | 359.3 | 1.1× |
| Double | 1 | 16.7 | 181.9 | **10.9×** |
| Double | 4 | 47.0 | 168.0 | **3.6×** |
| Double | 16 | 120.0 | 310.0 | **2.6×** |
| Double | 64 | 223.1 | 425.3 | **1.9×** |
| Double | 256 | 315.1 | 469.8 | 1.5× |
| Double | 1024 | 358.9 | 477.1 | 1.3× |
| Double | 4096 | 373.8 | 483.3 | 1.3× |
| String | 1 | 8.0 | 59.2 | **7.4×** |
| String | 4 | 21.7 | 57.9 | **2.7×** |
| String | 16 | 41.3 | 60.7 | **1.5×** |
| String | 64 | 62.9 | 80.6 | 1.3× |
| String | 256 | 75.2 | 82.0 | 1.1× |
| String | 1024 | 79.4 | 84.0 | 1.1× |
| String | 4096 | 80.4 | 83.4 | 1.0× |

Small batches (1–16): **2×–11×**. Large batches: no regression.

When the columns in the variant are sparse, during compaction, the
frequent add calls on the zonemap index cause field creation and field
comparison to account for around 12% of CPU usage. After optimization,
this drops to 3%.

None

- Test <!-- At least one of them must be included. -->
    - [ ] Regression test
    - [ ] Unit Test
    - [ ] Manual test (add detailed scripts or steps below)
    - [ ] No need to test or manual test. Explain why:
- [ ] This is a refactor/code format and no logic has been changed.
        - [ ] Previous test can cover this change.
        - [ ] No code files have been changed.
        - [ ] Other reason <!-- Add your reason?  -->

- Behavior changed:
    - [ ] No.
    - [ ] Yes. <!-- Explain the behavior change -->

- Does this need documentation?
    - [ ] No.
- [ ] Yes. <!-- Add document PR link here. eg:
apache/doris-website#1214 -->

- [ ] Confirm the release note
- [ ] Confirm test cases
- [ ] Confirm document
- [ ] Add branch pick label <!-- Add branch pick label that this PR
should merge into -->

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
(cherry picked from commit 555ea5c)