Disallow the legacy global session and allowlist existing call sites

[robert3005]

This changes LEGACY_SESSION top level lazy static into a method.

We also disallow this method from being used in the codebase and enshrine
existing call sites with exceptions

Signed-off-by: Robert Kruszewski github@robertk.io

[codspeed-hq]

Merging this PR will degrade performance by 10.63%

⚠️ 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.

⚡ 1 improved benchmark
❌ 3 regressed benchmarks
✅ 1591 untouched benchmarks
⏩ 4 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	chunked_varbinview_opt_canonical_into[(1000, 10)]	169.4 µs	206.2 µs	-17.84%
❌	Simulation	chunked_varbinview_into_canonical[(1000, 10)]	169.5 µs	205.6 µs	-17.55%
❌	Simulation	chunked_varbinview_opt_into_canonical[(1000, 10)]	183.3 µs	219.7 µs	-16.53%
⚡	Simulation	chunked_varbinview_into_canonical[(100, 100)]	306.7 µs	271.8 µs	+12.85%

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 claude/deprecate-legacy-session-au8j8j (c229b90) with develop (0026d3a)}

Footnotes

1. 4 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. ↩