Introduce SpanPrototype: baked-once constant span-tag descriptor

[dougqh]

What

Introduces SpanPrototype — a baked-once, immutable descriptor of a span's constant identity + tags (builder API: extends_ / init*), plus its per-mechanism JMH benchmark and a pure-API unit test.

Why

The constant tags a span carries are applied today via per-afterStart setTag calls. SpanPrototype bakes those constants once so a span can adopt them with a cheap copy instead. This PR lands the abstraction only — nothing constructs a span from a prototype yet.

Split of #11828

Part 1 of 2. This introduces the API in isolation; the decorator demo that bakes prototypes is the stacked follow-up, #11895. Construction (buildSpan(SpanPrototype)) is #11834 (dougqh/span-prototype-construction).

Notes

• SpanPrototype.NONE is intentionally kept: it's the no-prototype sentinel consumed by the construction PR (Thread SpanPrototype through span construction (parked until dense store) #11834 — CoreTracer default field + != NONE guard), not dead code.

🤖 Generated with Claude Code

[datadog-datadog-prod-us1-2]

🎯 Code Coverage (details)
• Patch Coverage: 60.47%
• Overall Coverage: 54.96% (-1.97%)

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[dd-octo-sts]

🟢 Java Benchmark SLOs — All performance SLOs passed

Suite	Status
Startup	🟢 pass

SLO thresholds are defined here based on automatically generated metrics. A warning is raised when results are within 5% of the threshold.

PR vs. master results

Scenario	Candidate	master	Δ (95% CI of mean)
startup:insecure-bank:iast:Agent	14.73 s	14.63 s	[-0.1%; +1.5%] (no difference)
startup:insecure-bank:tracing:Agent	13.61 s	13.67 s	[-1.2%; +0.3%] (no difference)
startup:petclinic:appsec:Agent	17.52 s	17.26 s	[+0.5%; +2.5%] (maybe worse)
startup:petclinic:iast:Agent	17.50 s	17.58 s	[-1.3%; +0.4%] (no difference)
startup:petclinic:profiling:Agent	17.47 s	17.51 s	[-1.2%; +0.7%] (no difference)
startup:petclinic:sca:Agent	17.58 s	17.53 s	[-0.6%; +1.2%] (no difference)
startup:petclinic:tracing:Agent	16.56 s	16.63 s	[-1.5%; +0.6%] (no difference)

Commit: 3af8e133 · CI Pipeline · Benchmarking Platform UI

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Load and DaCapo benchmarks can be triggered manually in the GitLab pipeline. Results will appear in the Benchmarking Platform UI after completion.