Native first reply: first() pays for 256 JSON-encoded rows, and the head of a columnar read is built in Python (left by #331) #381

Problem

On the native wheel, query(...).first() and every small result pay for up to 256 rows. The first reply of a query is JSON and carries up to _FIRST_BATCH = 256 rows: Java encodes them all in execImpl (native/src/main/java/org/humemai/nativelib/ArcadeNative.java:241, replyOf at :600) even when the caller wants one row, and NativeResultSet builds a deque(zip(rows, meta)) from them (src/arcadedb/_native/database.py:204-230). The JVM wheel reads one row for first() (RowAccess.firstAndClose). I read this at the tag 26.11.1.post2: #331 (native column frames, in post1 and post2) did not touch it.

#331 also left the head of a columnar read on the JSON path: a result of 256 rows or fewer never uses a column frame, and colframes._head_frame (src/arcadedb/_native/colframes.py:160-181) builds the head frame column by column in Python.

What is measured

The only timings are from 26.11.1, before #331, in the large-result study (native time over JVM time, above 1 means the native wheel is slower; only figures that pass the study's rule are listed):

These are baseline figures of 26.11.1 and are not measured on post2. I read the post2 code path as unchanged, so I expect the cost to be there, and I will measure it on post2 before claiming a size.

Work

• Encode the first reply lazily: for first() and one() ask for one row, and let a query that is read as a whole keep the batch.
• Retire colframes._head_frame: the head rows should arrive as a frame like the rest, so one decoder path covers a whole result.
• Add the missing test from Native: column frames for to_arrow, to_columns, to_dataframe and to_polars, read by the decoder the JVM wheel uses (refs #312) #331: a query that fails in the middle of a frame stream.
• Retire the old row path (_drain_dicts) once nothing calls it, so there is one path per type.

Method: a timed A/B of a candidate wheel against 26.11.1.post2 with an A/A control, on the figures above and on the small-result case, before any claim. Part of #315.