Emit low cardinality pageload span names #23519

Ensure pageload spans emit low-cardinality span names when span streaming is enabled.

Span name rules: https://getsentry.github.io/sentry-conventions/names/#browser-pageload

Fallback constant: PAGELOAD_SPAN_NAME_FALLBACK in packages/core/src/tracing/spans/spanNames.ts

Part of #22350 — remember to check traceLifecycle and keep the existing name for static mode.