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Experiment: avoid locks for empty xsi:type registries - #139
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Avoid registry read locks when a shared schema has no xsi:type variants. Keep the populated flag and registry update synchronized in the setter; cloned schemas share both. Add set/clear and concurrent-reader tests plus a matched empty/populated dispatch benchmark.
The experiment has a measured tradeoff. Three process pairs show empty-registry writes about 6.8% faster and tiny ordinary writes about 11 ns faster, while registered-variant reads are about 1.6% slower. Ordinary catalog reads are flat. Do not merge this on a broad parsing-speedup claim.
Retained evidence includes every Criterion sample, confidence interval, exact source patch, allocation diagnostics, Callgrind profiles, and unsuccessful borrowing/frame-size/fixed-guard experiments. See the experiment report.
Validation: 323 Rust tests, strict workspace formatting and Clippy, Ruff, and 112 Python tests with 100% statement/branch coverage.
Stacked on #137. Keep this draft for review of the workload tradeoff and synchronization cost.
Additional retained prototypes cover attribute guards and their combination with the registry flag: combined reads improve 3.5–4.1%, with mixed write tradeoffs. These prototypes are archived for review and are not part of this PR’s implementation.