Replace string-based error variants with structured fields

[ryanoneill]

Summary

• Replace Render(String), Config(String), Subscription(String) with named-field struct variants that expose component/field/type context
• Add convenience constructors EnvisionError::render(), ::config(), ::subscription() for ergonomic creation
• Update Display and Error::source() impls for the new structured variants
• Add field-accessibility tests to verify pattern matching works

Test plan

• cargo test — all 2885 tests pass
• cargo clippy -- -D warnings — clean
• cargo check --no-default-features — compiles
• cargo test --doc — 270 doc tests pass
• cargo build --examples — compiles

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

❌ Patch coverage is 93.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 92.64%. Comparing base (41832db) to head (ff008f3).

Files with missing lines	Patch %	Lines
src/error.rs	93.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #107      +/-   ##
==========================================
- Coverage   92.65%   92.64%   -0.02%     
==========================================
  Files          99       99              
  Lines        9491     9503      +12     
==========================================
+ Hits         8794     8804      +10     
- Misses        697      699       +2     

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