perf: stop allocating unused aggregate slots for every group

[melihbirim]

Replaces #181, which GitHub auto-closed when its base branch was deleted on merge of #180. Same branch, rebased onto main, now targeting main.

The problem

Every group's CompactAccum allocated three optional slot arrays — the COUNT(DISTINCT), MEDIAN and GROUP_CONCAT value buffers — whether or not the query used any of them.

sample on a single-threaded GROUP BY user_id over an 11.15 GB / 75M row fixture with 5M groups:

2134 of 3029 samples (70%)  engine.zig  CompactAccum.init
 861 of 3029 samples (28%)  hash_map    getOrPut

That run held 2810 MB for a 10-character key and one counter per group — 562 bytes per group.

The fix

A slot array that will only ever be read as null can be shared by the whole query instead of allocated per group. Reads still see null, and the branches that write one only run when the matching aggregate exists, in which case the array is allocated per group exactly as before. No use site changed.

GROUP BY user_id, 5M groups, 12 threads:

	memory
before	3395 MB
after	2188 MB

No regression on the paths that were already fast, same file:

query	after
GROUP BY country (8 groups)	2.27s / 55 MB
COUNT(*) WHERE country='JP'	2.01s / 53 MB
COUNT(DISTINCT user_id)	5.49s / 235 MB

Correctness

The risk is a shared array being written. Verified against DuckDB on a 2M-row file with all three optional kinds in one query, parallel and single-threaded:

SELECT country, COUNT(*), MEDIAN(amount), COUNT(DISTINCT device), GROUP_CONCAT(device)
  csvql   BR,250000,500.01,3,"tablet,desktop,..."
  duckdb  BR,250000,500.01,3

SELECT MEDIAN(amount), COUNT(DISTINCT device), COUNT(DISTINCT country)
  csvql   499.995,3,8
  duckdb  499.995,3,8

zig build test 638/638 after rebase, zig fmt --check clean.

What this does not fix

Memory only — wall clock on high-cardinality GROUP BY is unchanged. The parallel path replicates the group map per worker and merges serially, the same cliff #180 fixed for scalar COUNT(DISTINCT).

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