Enable experimental.varyParams by default (#94010)
Turns on `experimental.varyParams` by default. First of a short stack
flipping the remaining Sparkle flags to their new defaults — landing one
flag at a time so any regressions are bisectable.
A test in `segment-cache/prefetch-runtime` asserts on the pre-flip
root-param cache-keying; pinned to `varyParams: false` until the test
catches up.
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