Optimize dun_render FullyDark and FullyLit paths

[glebm]

Add overlapped_memset.hpp with FillBytesUpTo32, FillBytesUpTo64 (moved
from light_render.cpp), and CopyBytesUpTo32. All three avoid PLT/IFUNC
dispatch overhead for small variable-length memset/memcpy calls on Linux
by using fixed-size inline memcpy calls with overlapping stores/loads.

• RenderLineOpaque: use FillBytesUpTo32 instead of memset.
  Fixes ~3x slowdown vs PartiallyLit on the TransparentSquare RLE path.
• RenderLineOpaque: use CopyBytesUpTo32 instead of memcpy.
  TransparentSquare FullyLit was 6x slower than FullyDark despite doing
  less work per pixel; now on par with FullyDark (~5.8x improvement).
• Remove RenderTriangleLower/Upper<FullyDark, false> loop specializations.
  Falling back to the unrolled N-template lets the compiler see each blit
  width as a compile-time constant: FullyDark triangles ~5x faster,
  trapezoids ~2x faster, RenderBlackTile ~4x faster.
• RenderLineTransparentOrOpaqueN: for static N, bypass the fill/copy
  helpers and call BlitFillDirect (FullyDark) or BlitPixelsDirect (FullyLit)
  directly, guaranteeing a single vector instruction from the compiler.

Benchmarks (ns, RelWithDebInfo, single core, CPU scaling warnings apply):

Benchmark	Baseline	+FillUpTo32	-LoopSpecs	+CopyUpTo32
LTri So/FL	7266 ns	8377 ns	7745 ns	8427 ns
LTri So/FD	21506 ns	18274 ns	4151 ns	5758 ns
LTri So/PL	69289 ns	61874 ns	60053 ns	67337 ns
LTri Tr/FL	83602 ns	77991 ns	79224 ns	86274 ns
LTri Tr/FD	70809 ns	68356 ns	62336 ns	86026 ns
LTri Tr/PL	109707 ns	95830 ns	95321 ns	103802 ns
RTri So/FL	6220 ns	5953 ns	6052 ns	7392 ns
RTri So/FD	17101 ns	17778 ns	4145 ns	4878 ns
RTri So/PL	62913 ns	63671 ns	64209 ns	70015 ns
RTri Tr/FL	81890 ns	81554 ns	79492 ns	82832 ns
RTri Tr/FD	76801 ns	66247 ns	63267 ns	77821 ns
RTri Tr/PL	99755 ns	102136 ns	100410 ns	104500 ns
TrSq So/FL	322304 ns	340121 ns	330975 ns	56543 ns
TrSq So/FD	178437 ns	63454 ns	54919 ns	57429 ns
TrSq So/PL	127834 ns	141316 ns	131048 ns	147341 ns
TrSq Tr/FL	141366 ns	142851 ns	143384 ns	153830 ns
TrSq Tr/FD	125299 ns	121498 ns	130854 ns	133110 ns
TrSq Tr/PL	183519 ns	177058 ns	179739 ns	177087 ns
Sq So/FL	9618 ns	8923 ns	8291 ns	10151 ns
Sq So/FD	4885 ns	5449 ns	5426 ns	9530 ns
Sq So/PL	118292 ns	122309 ns	118639 ns	118512 ns
Sq Tr/FL	166114 ns	165816 ns	165845 ns	167127 ns
Sq Tr/FD	113779 ns	116176 ns	113845 ns	118415 ns
Sq Tr/PL	208899 ns	210274 ns	207931 ns	208100 ns
LTrap So/FL	2386 ns	2663 ns	2389 ns	2825 ns
LTrap So/FD	4159 ns	3879 ns	1723 ns	1924 ns
LTrap So/PL	31172 ns	30478 ns	30913 ns	31173 ns
LTrap Tr/FL	41760 ns	46841 ns	41622 ns	42224 ns
LTrap Tr/FD	31138 ns	31824 ns	32467 ns	33399 ns
LTrap Tr/PL	54863 ns	52467 ns	52177 ns	53973 ns
RTrap So/FL	2387 ns	2369 ns	2280 ns	3046 ns
RTrap So/FD	3901 ns	3772 ns	1522 ns	1914 ns
RTrap So/PL	29838 ns	30107 ns	30544 ns	34756 ns
RTrap Tr/FL	43348 ns	41696 ns	40635 ns	43537 ns
RTrap Tr/FD	32092 ns	31350 ns	31687 ns	31598 ns
RTrap Tr/PL	52474 ns	51980 ns	51348 ns	51540 ns
RenderBlackTile	98.7 ns	107 ns	21.2 ns	23.8 ns