fix!: canonicalize keys, update CI, and refresh dependencies

[AlexRiedler]

BREAKING CHANGE:

• Symbol hash keys are now canonicalized to strings internally. This affects #to_h, #to_hash, #inspect, and unmarshalled data that was originally created with symbol keys.
• Minimum supported Ruby version is now 3.2.
• StrictStruct#to_h / #to_hash now return a shallow copy instead of the internal mutable hash.

Other changes:

• Fix cache collisions in IndifferentLookup when a hash contains both symbol and string keys; cached nested structs now have stable identity.
• Fix README typos ("ResouceStruct") and GitHub URL placeholder.
• Remove unused bindir/executables from gemspec; add MFA metadata.
• Update CI matrix to Ruby 3.2, 3.3, 3.4, and 4.0.5; use actions/checkout@v4.
• Update CodeQL actions to v3.
• Update RuboCop to ~> 1.75 and rake to ~> 13.2; enable NewCops and target Ruby 3.2.
• Add regression tests for symbol/string key collisions and fix spec typos.