emnr: linear-time aepf, drop idiv from ring walks, halve getKey's logs

Three separate costs, found with a sampling profile of the RX chain with
emnr forced on:

aepf() averaged mask[] over a window of N = 2*psi + 1 = 41 bins by walking
the window for every one of the 2049 bins, i.e. O(msize*N). Its three spans
are all symmetric windows clipped at the array ends, so take each from a
prefix sum instead: one subtraction per output, O(msize).

xemnr() advanced four ring indices with a '% size' per step. iasize is 4096
and oasize 1024 here, and neither is known to the compiler, so each step was
a real integer division -- ~8700 of them per frame. The indices step by one
and, since iasize >= fsize and oasize >= incr always hold, wrap at most once
per loop, so walk contiguous runs and wrap between them.

calc_gain() called getKey() twice per bin with the same gamma, so the gamma
row index and its log10 were computed twice. Split getKey into keyIndex() +
keyLerp() and locate gamma once. The remaining logs go through wdsp_log10()
(new fastmath.h), accurate to 2e-13 against libm and ~2.4x its throughput;
gamma and xi are bracketed against the table limits first, so the argument
is always positive and normal. Also clamp the row index so the second
bilinear corner cannot address the next row of the 241x241 table.

Measured in situ on an Apple M1 Pro, 512-sample buffers, cost of turning
emnr on, best of 5:

    baseline                       73661 ns
    + aepf, ring walks             48927 ns   1.51x
    + getKey                       37852 ns   1.95x

Output is not bit-identical, as the prefix sum and the reassociated logs
round differently. Over 300 buffers with emnr alone the worst deviation is
4.0e-09, an SNR of 196 dB; perturbing a single input sample of the unmodified
code by one ulp diverges it from itself by 1.4e-08 (186 dB), so this change
disturbs the chain less than the last bit of the input does.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

emnr.h

@@ -185,6 +185,7 @@ typedef struct _emnr
 		double zetaThresh;
 		double psi;
 		double* nmask;
+		double* csum;		// prefix sums of mask[], msize + 1 entries
 		double t2;
 	} ae;
 		struct _post2