Fix Safari dormant overhead in Live Debugger instrumentation

[watson]

What and why?

The Live Debugger runtime benchmark surfaced a large dormant-probe overhead on Safari: a tiny instrumented function cost ~8.9 ns/call (~1,100% over baseline) versus ~0 ns on Chrome and ~1.6 ns on Firefox. The root cause is that the transform hoisted each function's captured parameters into a per-call arrow closure (const $dd_eN = () => ({...})). A nested closure that captures parameters forces JavaScriptCore — Safari and all iOS browsers, since they're all WebKit — to heap-allocate the function's scope on every invocation, even when probes are dormant and the closure is never created or called. V8 and SpiderMonkey sink that allocation; JavaScriptCore does not, so it dominated the dormant overhead. Because every instrumented iOS app pays this cost on every call, it's worth eliminating.

How?

Inline the captured-arguments object literal ({a, b}) directly at each probe-guarded call site instead of hoisting it into a per-call closure. The object now lives inside the if (probe) / probe ? guards, so dormant calls allocate nothing; gating the closure behind the probe check was confirmed insufficient because the captured scope is still allocated. The active path is unchanged — the same arguments object is built the same number of times when a probe fires. Verified with the runtime benchmark: Safari per-call dormant overhead drops ~73% (8.9 → 2.4 ns) and the Hot workload from ~10 to ~7 ns, with no change on Chrome or Firefox. Transform unit-test snapshots and the plugin README/EXAMPLES output catalog are updated to match the new generated shape.

[watson]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• Add decorator support to Live Debugger instrumentation #448
• Fix Live Debugger parenthesized arrow body instrumentation #447
• Fix Live Debugger returned arrow instrumentation #442
• Fix live debugger curried function instrumentation #441
• Fix Safari dormant overhead in Live Debugger instrumentation #438 👈 (View in Graphite)
• Add Live Debugger runtime benchmark coverage #429 : 1 other dependent PR (#440 )
• master

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[datadog-official]

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[github-actions]

Live Debugger Runtime Benchmark

SDK-loaded dormant-probe runtime overhead, measured against an uninstrumented bundle in the same browser session.

Browser	Workload	Quality	Per-call overhead upper
chrome	Hot	clean	<= 2.95 ns
chrome	Tiny	clean	<= 0.02 ns
firefox	Hot	clean	<= 7.77 ns
firefox	Tiny	clean	<= 0.27 ns
safari	Hot	clean	<= 4.46 ns
safari	Tiny	clean	<= 1.10 ns

Full diagnostics

browser  workload  quality  per-call overhead upper  overhead upper          95% CI        A/A diag        block CI  acf(1)   baseline  instrumented                         samples
-------  --------  -------  -----------------------  --------------  --------------  --------------  --------------  ------  ---------  ------------  ------------------------------
chrome   Hot       clean                 <= 2.95 ns        <= 6.83%   2.94..2.95 ns  -0.01..0.01 ns   2.94..2.95 ns   -0.00  51.225 ms     54.720 ms   102 (trim 20%, outliers 9.8%)
chrome   Tiny      clean                 <= 0.02 ns        <= 0.35%  -0.02..0.02 ns  -0.05..0.02 ns  -0.02..0.02 ns   -0.04  55.290 ms     55.310 ms  102 (trim 20%, outliers 12.7%)
firefox  Hot       clean                 <= 7.77 ns       <= 36.50%   7.76..7.77 ns  -0.01..0.00 ns   7.76..7.77 ns   -0.40  40.450 ms     55.200 ms   102 (trim 20%, outliers 8.8%)
firefox  Tiny      clean                 <= 0.27 ns        <= 2.29%   0.27..0.27 ns  -0.00..0.00 ns   0.27..0.27 ns    0.08  50.780 ms     51.940 ms   102 (trim 20%, outliers 5.9%)
safari   Hot       clean                 <= 4.46 ns       <= 21.18%   4.45..4.46 ns  -0.00..0.00 ns   4.45..4.46 ns    0.17  45.460 ms     55.040 ms   102 (trim 20%, outliers 3.9%)
safari   Tiny      clean                 <= 1.10 ns       <= 33.55%   1.09..1.10 ns  -0.00..0.00 ns   1.09..1.10 ns   -0.13  41.180 ms     55.000 ms   102 (trim 20%, outliers 7.8%)

Raw samples are in the live-debugger-runtime-bench-results artifact.