vortex-row: RowEncode ScalarFn

[joseph-isaacs]

Part 7 of 25 in the stacked PR series adding vortex-row.

This PR contains exactly one commit; review just that diff in isolation.

What this commit does

Adds the RowEncode variadic scalar function: encode N input columns into a single ListView in a five-phase pipeline.

• Phase 1: size pass via compute_sizes.
• Phase 2: allocate a zero-initialized output buffer sized to fit every row's encoded bytes; bail if the total exceeds u32::MAX.
• Phase 3: build per-row listview_offsets via i * fixed_per_row (pure-fixed) or i * fixed_per_row + exclusive cumsum of varlen lengths. Uses the simple Vec::push + checked_add loop.
• Phase 4: walk columns left-to-right and call dispatch_encode for every column. Each call writes its per-row bytes at offsets[i] + cursors[i] and advances the cursor.
• Phase 5: build the ListView via the validating try_new constructor.

dispatch_encode is the canonicalize-then-codec::field_encode fallback; in-crate kernel arms and the inventory registry land in PR 3. PR 2 iterates on this pipeline (skip zero-init, skip ListView validation, auto-vectorize the offsets loop, etc.).

Stack

#	PR	Title	Branch
1	#7986	vortex-row: crate scaffolding	claude/row-c01-crate-scaffolding
2	#7987	vortex-row: add SortField and RowEncodeOptions	claude/row-c02-sortfield-options
3	#7988	vortex-row: codec for fixed-width canonical types	claude/row-c03-codec-fixed-width
4	#7989	vortex-row: codec for varlen canonical types	claude/row-c04-codec-varlen
5	#7990	vortex-row: codec for nested canonical types	claude/row-c05-codec-nested
6	#7991	vortex-row: compute_sizes helper and RowSize ScalarFn	claude/row-c06-rowsize-scalarfn
7	#7992	vortex-row: RowEncode ScalarFn	claude/row-c07-rowencode-scalarfn
8	#7993	vortex-row: convert_columns + tests + bench scaffolding	claude/row-c08-convert-columns-tests-bench
9	#7994	Skip ListView validation in row encoder output	claude/row-c09-skip-listview-validation
10	#7995	Add validity fast-path helper for the four pattern-matching encoders	claude/row-c10-validity-fast-path
11	#7996	Skip zero-init of output buffer	claude/row-c11-skip-zero-init
12	#7997	Auto-vectorize pure-fixed offsets construction	claude/row-c12-vectorize-pure-fixed-offsets
13	#7998	Auto-vectorize mixed-path offsets construction	claude/row-c13-vectorize-mixed-offsets
14	#7999	Rewrite varlen 32-byte block encoder with copy_nonoverlapping	claude/row-c14-varlen-block-copy-nonoverlapping
15	#8000	Walk VarBinView rows directly in row encoder hot loop	claude/row-c15-walk-varbinview-directly
16	#8001	Add arithmetic-write fast path for fixed-before-varlen columns	claude/row-c16-arith-write-fast-path
17	#8002	Specialize Constant for the arithmetic-write fast path	claude/row-c17-specialize-constant-arith
18	#8003	RowSizeKernel and RowEncodeKernel dispatch helpers	claude/row-c18-kernel-dispatch-helpers
19	#8004	Inventory-based registry for downstream encoding kernels	claude/row-c19-inventory-registry
20	#8005	Constant row-encode kernel	claude/row-c20-constant-kernel
21	#8006	Dict row-encode kernel	claude/row-c21-dict-kernel
22	#8007	Patched row-encode kernel	claude/row-c22-patched-kernel
23	#8008	RunEnd row-encode kernel (vortex-runend)	claude/row-c23-runend-kernel
24	#8009	BitPacked row-encode kernel (vortex-fastlanes)	claude/row-c24-bitpacked-kernel
25	#7985	FoR and Delta row-encode kernels (vortex-fastlanes)	claude/row-pr3-kernels

Base of this PR: #7991 (claude/row-c06-rowsize-scalarfn)
Next in stack: #7993 (claude/row-c08-convert-columns-tests-bench)

Combined context

For the full design + rationale, see PR #7985 (top of stack).

[codspeed-hq]

Merging this PR will improve performance by 14.29%

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

⚡ 3 improved benchmarks
✅ 1218 untouched benchmarks

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	new_alp_prim_test_between[f32, 16384]	118.3 µs	103.8 µs	+13.96%
⚡	Simulation	new_bp_prim_test_between[i16, 32768]	132.3 µs	120.1 µs	+10.23%
⚡	Simulation	new_alp_prim_test_between[f32, 32768]	182.1 µs	153.2 µs	+18.85%

Tip

Curious why this is faster? Comment @codspeedbot explain why this is faster on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing claude/row-c07-rowencode-scalarfn (40783a6) with claude/row-c06-rowsize-scalarfn (5374f3b)}