Adds tests/native/test_lexer_scaling.el, the regression gate for el #132.
Both directions are proven on LIVE workloads, not synthetic series:
healthy per-character scan 1821 3251 6007 10422 us -> O(n) PASS
rescan-from-zero (the #132 shape) 922 3667 13524 44792 -> O(n^2) FAIL
A gate only proven to pass is decoration. The quadratic specimen exists so
the gate is proven to FIRE.
Also fixes elb_spread_ok to judge the ASYMPTOTIC TAIL (last three ratios)
rather than the whole sweep. Measured on a genuinely linear scan the ratios
ran 3.37 2.92 1.76 1.65 -- the head looks quadratic because it is cold
cache, the tail is the truth. Whole-sweep spread rejected correct data. A
complexity bound is an asymptotic claim and must be judged asymptotically.
That fix came from the classifier refusing to rubber-stamp my own bad
measurement: it reported INDETERMINATE on an unwarmed sweep rather than
passing it. Warmup is now taken and discarded at every sweep point.
Reverts the == workarounds in test_elbench.el now that el #137 has landed;
the natural form generates no str_eq and all 13 fitter tests stay green.
The workaround remains -- the Plus arm is still open.
Neuron