Add strict duplicate-key parsing and JuliaC coverage

[quinnj]

Summary

• add duplicate_keys=:error and JSON.DuplicateKeyError while preserving the default last-value-wins behavior
• reject decoded-key collisions, including escaped keys, nested objects, JSON Lines, and typed parsing
• add an isolated JuliaC --trim=safe workload that verifies public read and write entry points and runs the produced executable
• prepare version 1.7.0

Motivation

OpenAPI document loaders must reject duplicate object keys instead of silently accepting ambiguous input. Downstream applications also need JSON.jl to remain compatible with JuliaC trim compilation.

Validation

• duplicate-key regression coverage includes ordinary, nested, escaped, JSON Lines, typed, and invalid-option cases
• the trim harness requires zero verifier errors and zero warnings, then executes the generated binary
• the default duplicate-key behavior remains unchanged

Co-authored by Codex

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.48%. Comparing base (e5ef310) to head (13f8107).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #474      +/-   ##
==========================================
+ Coverage   90.41%   90.48%   +0.07%     
==========================================
  Files           7        7              
  Lines        1419     1430      +11     
==========================================
+ Hits         1283     1294      +11     
  Misses        136      136              

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