Fix PtrString hash compatibility

[quinnj]

Summary

• Add reverse AbstractString/PtrString equality so comparisons are symmetric.
• Add Base.hash(::PtrString, ::UInt) so equal PtrString and String values hash identically.
• Cover the regression with lazy parser tests for equality, hashing, Dict, and Set lookup.

Root Cause

PtrString defined ==/isequal against AbstractString but did not define a matching hash method. Hash-table lookup also exposed that the equality methods only handled PtrString on the left-hand side.

Validation

• tmpenv=$(mktemp -d); julia --startup-file=no --project="$tmpenv" -e 'using Pkg; Pkg.develop(path=pwd()); using JSON, Test; include("test/lazy.jl")'
• tmpenv=$(mktemp -d); julia --startup-file=no --project="$tmpenv" -e 'using Pkg; Pkg.develop(path=pwd()); Pkg.test("JSON")'

Fixes #467

Co-authored by Codex

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.41%. Comparing base (6e487a9) to head (7b61d20).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #468      +/-   ##
==========================================
+ Coverage   90.32%   90.41%   +0.09%     
==========================================
  Files           7        7              
  Lines        1416     1419       +3     
==========================================
+ Hits         1279     1283       +4     
+ Misses        137      136       -1     

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