fix: md/raw switch alignment, and effect 4.0.4 span changes breaking CI - #1450

🟢 Confidence 8/10 · likely safe to merge
The 5xx rule is narrow and pinned, but the Effect 4.0.2 HttpServerError/ResponseError shape is only mirrored by a hand-built fixture, never a real router round trip.
_{quality 100/100 · no findings · tests covered · risk medium}

Two unrelated threads: ViewSwitch is pinned to the top of the system-prompt bodies (self-start), and the SDK tracer now treats Effect's own HttpServerError/ResponseError for a 5xx as the rendered 5xx. Both are contained; the tracer branch is narrowly gated and tested.

• ViewSwitch takes self-start at the two body-level call sites
• makeOtlpSpan accepts an HttpServerError/ResponseError failure for a 5xx as renderedServerError
• Worker-bridge and warehouse span tests now select server spans by kind
• New test pins HttpServerErrorResponse on a failed 5xx span

_{4d3b5b2 · Updated on every push. Reply "won't fix" to dismiss a finding, or mention @maple-review-bot to ask about one.}