cd5f21e9

By: ValentinHirschi <valentin.hirschi@gmail.com>

checkpoint experimental four-loop rustred comparisons

Preserve the opt-in experimental native scalar bridge, shared numerator
lowering, finite-target FMFT comparisons, exact-before-approximate finalizer
fix, and cold/warm timing evidence. Pin the already-pushed RustRed d686d09
revision. Production defaults remain unchanged; this is not a shipped certified
four-loop backend. The mandatory ci-checks-and-upload recipe is absent on this
branch, so this commit remains local until that publication gate is available.

User checkpoint request (verbatim):

I would like you to continue exploring all the lanes stated in the goal (read it again) but you won't be able to complete it with my current quota available.
So I would like you to organize the work to get to a clean status (committed+push) on all the aspects mentioned in the goal and summarize to me where everything stands (vakint 4L FORM-lesss numerical tests reproduction, and performance vs FMFT, rustred vs Spider performance, etc..)
Then pause the goal and stop yourself; Aim for managing to complete this "checkpoint" of your progress in about one hour. So no new feature development for now, but really aim for a stable and usable snapshot of current progress.

User report request (verbatim):

(continue as planned, but what is this rustred cache you're talking about? Because if it can be used live during gammaloop generation when building the integrated UV CTs, and the warm-cache time is 34ms alone, then it may be fine and acceptable).
Once you have completed this snapshot and before pausing yourself, write all current status in a detailed report `STATUS_17_09_2026.md` (aim for 5-6 pages, not 20).

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