src: use stack allocation for small string encoding

[thisalihassan]

• Introduce EncodeOneByteString / EncodeTwoByteString helper templates in StringBytes::Encode() that use a 512 byte (256-char for two-byte) stack buffer for small inputs, falling back to heap allocation + ExternString for larger ones.
• Covers all encoding paths: ASCII, HEX, BASE64, BASE64URL, UTF8 (two-byte), and UCS2.

Refs: nodejs/performance#194

                                                                       confidence improvement accuracy (*)   (**)  (***)
buffers/buffer-base64-encode.js n=32 len=67108864                                      1.82 %       ±2.53% ±3.38% ±4.42%
buffers/buffer-base64url-encode.js n=32 len=67108864                            *     -1.70 %       ±1.39% ±1.86% ±2.42%
buffers/buffer-tostring.js n=1000000 len=1 args=0 encoding=''                 ***      3.71 %       ±1.67% ±2.23% ±2.91%
buffers/buffer-tostring.js n=1000000 len=1 args=1 encoding='ascii'                     0.21 %       ±2.08% ±2.78% ±3.65%
buffers/buffer-tostring.js n=1000000 len=1 args=1 encoding='hex'              ***     40.34 %       ±4.17% ±5.60% ±7.39%
buffers/buffer-tostring.js n=1000000 len=1 args=1 encoding='latin1'           ***      2.52 %       ±1.15% ±1.53% ±1.99%
buffers/buffer-tostring.js n=1000000 len=1 args=1 encoding='UCS-2'            ***      3.33 %       ±1.26% ±1.68% ±2.19%
buffers/buffer-tostring.js n=1000000 len=1 args=1 encoding='utf8'             ***      2.03 %       ±0.90% ±1.19% ±1.55%
buffers/buffer-tostring.js n=1000000 len=1 args=3 encoding='ascii'              *      3.00 %       ±2.83% ±3.80% ±5.02%
buffers/buffer-tostring.js n=1000000 len=1 args=3 encoding='hex'              ***     40.40 %       ±3.23% ±4.30% ±5.59%
buffers/buffer-tostring.js n=1000000 len=1 args=3 encoding='latin1'            **      2.93 %       ±2.04% ±2.73% ±3.58%
buffers/buffer-tostring.js n=1000000 len=1 args=3 encoding='UCS-2'            ***      4.55 %       ±1.17% ±1.56% ±2.04%
buffers/buffer-tostring.js n=1000000 len=1 args=3 encoding='utf8'             ***      2.52 %       ±0.88% ±1.17% ±1.52%
buffers/buffer-tostring.js n=1000000 len=1024 args=0 encoding=''                      -0.48 %       ±1.75% ±2.35% ±3.09%
buffers/buffer-tostring.js n=1000000 len=1024 args=1 encoding='ascii'                  2.68 %       ±3.68% ±4.91% ±6.44%
buffers/buffer-tostring.js n=1000000 len=1024 args=1 encoding='hex'                   -0.05 %       ±1.47% ±1.96% ±2.55%
buffers/buffer-tostring.js n=1000000 len=1024 args=1 encoding='latin1'          *      2.53 %       ±1.94% ±2.60% ±3.41%
buffers/buffer-tostring.js n=1000000 len=1024 args=1 encoding='UCS-2'                  0.75 %       ±1.80% ±2.40% ±3.13%
buffers/buffer-tostring.js n=1000000 len=1024 args=1 encoding='utf8'            *      3.19 %       ±2.47% ±3.33% ±4.40%
buffers/buffer-tostring.js n=1000000 len=1024 args=3 encoding='ascii'                  0.01 %       ±2.15% ±2.86% ±3.72%
buffers/buffer-tostring.js n=1000000 len=1024 args=3 encoding='hex'                    0.40 %       ±1.31% ±1.74% ±2.26%
buffers/buffer-tostring.js n=1000000 len=1024 args=3 encoding='latin1'          *      1.96 %       ±1.86% ±2.48% ±3.23%
buffers/buffer-tostring.js n=1000000 len=1024 args=3 encoding='UCS-2'                  0.54 %       ±1.99% ±2.65% ±3.46%
buffers/buffer-tostring.js n=1000000 len=1024 args=3 encoding='utf8'                   1.05 %       ±1.11% ±1.48% ±1.93%
buffers/buffer-tostring.js n=1000000 len=64 args=0 encoding=''                        -0.37 %       ±1.28% ±1.72% ±2.26%
buffers/buffer-tostring.js n=1000000 len=64 args=1 encoding='ascii'                   -1.33 %       ±2.21% ±2.97% ±3.92%
buffers/buffer-tostring.js n=1000000 len=64 args=1 encoding='hex'             ***     41.64 %       ±2.07% ±2.76% ±3.62%
buffers/buffer-tostring.js n=1000000 len=64 args=1 encoding='latin1'            *      1.98 %       ±1.86% ±2.48% ±3.25%
buffers/buffer-tostring.js n=1000000 len=64 args=1 encoding='UCS-2'                   -0.38 %       ±2.76% ±3.68% ±4.80%
buffers/buffer-tostring.js n=1000000 len=64 args=1 encoding='utf8'                     0.64 %       ±0.88% ±1.18% ±1.53%
buffers/buffer-tostring.js n=1000000 len=64 args=3 encoding='ascii'                    0.35 %       ±2.91% ±3.88% ±5.07%
buffers/buffer-tostring.js n=1000000 len=64 args=3 encoding='hex'             ***     40.54 %       ±1.43% ±1.90% ±2.48%
buffers/buffer-tostring.js n=1000000 len=64 args=3 encoding='latin1'                   1.67 %       ±2.47% ±3.30% ±4.31%
buffers/buffer-tostring.js n=1000000 len=64 args=3 encoding='UCS-2'                    0.32 %       ±1.93% ±2.57% ±3.35%
buffers/buffer-tostring.js n=1000000 len=64 args=3 encoding='utf8'                     0.56 %       ±0.91% ±1.21% ±1.58%

Be aware that when doing many comparisons the risk of a false-positive
result increases. In this case, there are 35 comparisons, you can thus
expect the following amount of false-positive results:
  1.75 false positives, when considering a   5% risk acceptance (*, **, ***),
  0.35 false positives, when considering a   1% risk acceptance (**, ***),
  0.04 false positives, when considering a 0.1% risk acceptance (***)
thisalihassan@Alis-MacBook-Pro node % 

[codecov]

Codecov Report

❌ Patch coverage is 75.43860% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.69%. Comparing base (0d7e4b1) to head (43fd2f8).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/string_bytes.cc	75.43%	5 Missing and 9 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #62431   +/-   ##
=======================================
  Coverage   89.68%   89.69%           
=======================================
  Files         676      676           
  Lines      206738   206748   +10     
  Branches    39594    39594           
=======================================
+ Hits       185414   185436   +22     
+ Misses      13467    13462    -5     
+ Partials     7857     7850    -7     

Files with missing lines	Coverage Δ
src/string_bytes.cc	73.17% <75.43%> (+3.50%)	⬆️

... and 32 files with indirect coverage changes

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