inky: fix blend precision, height quirk, and minor cleanups

[caglar10ur]

• Fix color blending truncation in blend(): compute the full weighted sum as float64 before casting to uint8, avoiding double truncation
• Fix UC8159 on IMPRESSION57 incorrectly reporting height as 447; correct it to 448 in DetectOpts
• Add Opts.String() for human-readable option printing
• Refactor update() if/else chain to a switch statement
• Add TestDevImpression_BlendPrecision to cover the blend fix

[codecov]

Codecov Report

❌ Patch coverage is 30.76923% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 52.7%. Comparing base (1752d1b) to head (89a9731).

Files with missing lines	Patch %	Lines
inky/impression.go	44.4%	5 Missing ⚠️
inky/opts.go	0.0%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #117     +/-   ##
=======================================
+ Coverage   52.6%   52.7%   +0.1%     
=======================================
  Files        117     117             
  Lines      11801   11803      +2     
=======================================
+ Hits        6205    6220     +15     
+ Misses      5232    5218     -14     
- Partials     364     365      +1     

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