test(compat): add a map.vortex backward-compatibility fixture

[mprammer]

Rationale for this change

vortex.map joined the frozen core2026.08.0 edition at Vortex 0.84.0, which makes its layout a backward-compatibility promise, but the compat fixture suite has no map file — nothing pins how a map is written, so the weekly compat-validation run has nothing to check and a future reader change could silently break every map file written today. The gap isn't map-specific: vortex.masked (frozen since 0.54.0) and vortex.variant (frozen since 0.65.0) also have no fixtures, and I'll send those separately.

What changes are included in this PR?

One new fixture, map.vortex, in the synthetic array set. It carries a map<i32, utf8?> with keys_sorted = false including an empty row and a row that repeats a key, since duplicate keys are representable and have to survive in order rather than being deduplicated; a map<utf8, i64> with keys_sorted = true; and a nullable map<i32, utf8?> that keeps a null map distinct from an empty one and holds a present key with a null value. The existing roundtrip_fixtures_to_bytes test picks the fixture up automatically, and vortex-compat generate followed by check --mode exact passes on all 36 fixtures. Worth flagging for review that build() is immutable once published, so this content can be added to later but not amended — the cases are the part to scrutinise.

🤖 Generated with Claude Code

[codspeed-hq]

Merging this PR will degrade performance by 16.08%

⚠️ 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 regressed benchmarks
✅ 2001 untouched benchmarks
⏩ 89 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	decompress[u64, (10000, 4)]	310.2 µs	401.6 µs	-22.75%
❌	Simulation	cold_misaligned[(64, 256)]	4.4 ms	5 ms	-12.78%
❌	WallTime	words_gather_scalar[65536]	8.3 µs	9.4 µs	-12.28%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing mp/compat-fixture-map (365b8dd) with develop (004e512)}

Footnotes

1. 89 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩