Drop redundant deep-copy of UDF return values (~3-16% perf on UDFs in mlr put) (#2089)

* Batch-allocate per-record objects; reuse CSV writer field buffer

After batch-arena field allocation, profiling cat over 1M-record CSV showed
the remaining ~5M allocations were almost entirely per-record (one each):
the Mlrmap struct, the RecordAndContext wrapper, the CSV writer's []string,
and the go-csv parser's own buffers.

Address the first three:

- mlrval.RecordArena gains NewRecord(), vending the Mlrmap struct itself from
  a per-batch slab (respecting --no-hash-records). Rolled out to every
  line-based reader (CSV, CSV-lite, TSV, DKVP, NIDX, PPRINT, XTAB, DKVPX) in
  place of NewMlrmapAsRecord.

- The CSV reader batch-allocates RecordAndContext wrappers from a per-batch
  slab instead of one heap object per record (comment/output-string entries
  still allocate individually, but they are rare).

- RecordWriterCSV reuses a single fieldsBuffer []string across records instead
  of allocating one per Write; WriteCSVRecordMaybeColorized consumes it
  synchronously and the writer is single-goroutine, so this is safe.

Effect (big.*, 1M records, cat, best of 5):
  csv   0.26 -> 0.22
  dkvp  0.51 -> 0.45  (Mlrmap slab)

For CSV, cat's allocation-object count drops ~5.0M -> ~2.1M. The remaining
~2M are the go-csv parser's per-record backing string and field slice, which
are intrinsic to parsing and would require a zero-copy/batch-slab parser
rework. A CPU profile of cat now shows it is I/O-bound (syscall ~56%, bufio
read+flush), with allocation/GC down to ~10% -- i.e. further allocation
trimming no longer moves cat's wall-clock. GOGC=off confirms (no change).

Verified: go test ./pkg/... and full regression suite pass; output is
byte-identical across all formats including record-retaining verbs (tac),
hashed and --no-hash-records.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* Pool DSL stack frames across records (~8-9% on put)

A StackFrameSet lives on the persistent runtime.State and is reused across
all records, but every block entry (StatementBlockNode.Execute does
PushStackFrame/PopStackFrame, which runs once per record for the main block,
plus once per if/for/etc.) allocated a fresh StackFrame -- a []*var slice and
a map[string]int -- and discarded it on exit. For `put`/`filter` that is
millions of throwaway allocations.

Since push/pop is strictly LIFO, retain popped frames in a per-frameset free
list and clear-and-reuse them on the next push. After the first record
establishes the max block-nesting depth, per-record block execution is
allocation-free for frames. len(stackFrames) remains the logical depth, so
get/set/defineTyped/unset/etc. are unchanged.

Measured (big.csv, 1M rows, best of 4):
  put  chain-1  0.78 -> 0.72  (~8%)
  put  chain-4  0.96 -> 0.87  (~9%)

Allocation objects for put chain-1 drop ~23.1M -> ~20.0M (the per-record
newStackFrame churn, ~2.86M, is eliminated). UDF calls still allocate a fresh
frameset per call (PushStackFrameSet); pooling those is a separate change.

The dominant remaining DSL allocator is FromFloat (~6.8M, interior arithmetic
temporaries); eliminating it needs node-owned result slots + in-place bif
variants, a much larger and aliasing-sensitive change, left for follow-up.

Verified: go test ./pkg/... and full regression suite pass; put output is
byte-identical, including UDFs with locals/loops/blocks.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* Pool DSL stack-frame *sets* across UDF/subr calls (~31% on function-heavy put)

Companion to the per-block frame pooling: that left PushStackFrameSet /
PopStackFrameSet (entered once per user-defined function or subroutine call)
allocating. Each call did newStackFrameSet() -- a StackFrameSet plus its
initial StackFrame (a slice and a map) -- AND, worse, prepended it with
append([]*StackFrameSet{head}, sets...), allocating a fresh backing slice and
copying the whole save-stack every call.

Two changes:
  - Treat the frameset save-stack as a tail stack (append to push, truncate to
    pop) instead of prepending at index 0. get/set only ever touch the cached
    head, so list order is irrelevant; this removes the per-call slice
    realloc + O(depth) copy.
  - Pool popped framesets (LIFO) and reset-and-reuse them on the next push,
    mirroring the per-frameset frame free list. A reset trims back to one
    cleared base frame (extras go to the frame pool). After warmup, repeated
    calls allocate no framesets or frames.

Measured (big.csv, 1M rows, best of 5):
  put, 2 nested func calls/record:  2.73 -> 1.87  (~31%)
GC cycles 25 -> 16; newStackFrameSet/newStackFrame fall out of the allocation
profile entirely. (chain-1 etc. have no UDFs and are unaffected.)

Verified: go test ./pkg/... and full regression suite pass; recursion
(fact/fib), local-scope isolation, and subroutine+oosvar all correct.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* Drop redundant deep-copy of UDF return values (~3-16% on UDF put)

A user-defined function's return value was deep-copied twice on the way out:
once in ReturnNode.Execute (returnValue.Copy() when building the block-exit
payload) and again in UDFCallsite.EvaluateWithArguments
(blockReturnValue.Copy() at the end).

The ReturnNode copy is the necessary one: it detaches the value from the
callee's frame so it survives the frame being popped (and, since perf-try-7,
pooled/reused). By the time EvaluateWithArguments returns, blockReturnValue is
therefore already an independent deep copy, so the second copy is pure waste --
and callers that retain the result copy again anyway (field/oosvar/local
assignment all PutCopy/Copy). The other return paths (implicit-absent, error)
don't use blockReturnValue, so this only affects the BLOCK_EXIT_RETURN_VALUE
path.

Return blockReturnValue directly.

Measured (big.csv, 1M rows, best of 5):
  put, 2 nested scalar-returning calls/record:  1.89 -> 1.83  (~3%)
  put, map-returning func per record:           2.34 -> 1.97  (~16%)
Win scales with return-value size (the avoided copy is deep). All UDF/HOF
callsites (apply/reduce/sort/select/fold/...) go through this path.

Verified: go test ./pkg/... and full regression suite pass; recursion, HOFs,
and returned-map isolation (mutating a returned map does not affect a
subsequent call) all correct.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

pkg/dsl/cst/udf.go

@@ -304,7 +304,12 @@ func (site *UDFCallsite) EvaluateWithArguments(
 		"function "+udf.signature.funcOrSubrName+" return value",
 	)
 
-	return blockExitPayload.blockReturnValue.Copy()
+	// blockReturnValue is already an independent deep copy: ReturnNode.Execute
+	// does returnValue.Copy() when building the payload, precisely so the value
+	// survives the callee frame being popped (and, with frame pooling, reused).
+	// A second copy here is redundant -- the caller copies again if it stores
+	// the value (field/oosvar/local assignment all PutCopy/Copy).
+	return blockExitPayload.blockReturnValue
 }
 
 // UDFManager tracks named UDFs like 'func f(a, b) { return b - a }'