Add embedded struct support with optimized fast path

[shamaton]

• Add a shared field collector that expands embedded structs, resolves name conflicts, and mirrors encoding/json behavior.
• Update map/array encode/decode (including stream) to use field paths, support embedded pointers, and honor parent omitempty.
• Add comprehensive tests for embedded structs, tags, ambiguity, and nil pointer cases to align behavior with JSON.

[codecov]

Codecov Report

❌ Patch coverage is 88.56589% with 59 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.36%. Comparing base (f71f5e2) to head (984f35f).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/stream/decoding/struct.go	80.50%	17 Missing and 6 partials ⚠️
internal/decoding/struct.go	83.05%	16 Missing and 4 partials ⚠️
internal/common/common.go	92.59%	5 Missing and 1 partial ⚠️
internal/stream/encoding/struct.go	92.30%	3 Missing and 3 partials ⚠️
internal/encoding/struct.go	96.69%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #58      +/-   ##
==========================================
- Coverage   99.26%   98.36%   -0.90%     
==========================================
  Files          72       72              
  Lines        5677     6038     +361     
==========================================
+ Hits         5635     5939     +304     
- Misses         29       70      +41     
- Partials       13       29      +16     

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