Make model fields private with Bon builders

[Sajjon]

Summary

• add Bon 3.9.3 and compile-time-checked builders for exposed model construction
• make model data fields private, with read-only accessors or intentional consuming APIs
• add getset 0.1.7 proc-macro derives for every simple field accessor in the core library and CLI, including copy and clone getters where needed
• add targeted CLI and model tests for explicit-path conversion, missing-file errors, builder construction, answer replacement, empty answers, and consuming APIs
• correct the sealed-file read error mapping to FailedToReadSecretFromFile
• clean up a pre-existing redundant map closure required by the repository Clippy hook

Verification

• cargo test --workspace --all-targets (93 tests)
• cargo test --workspace --doc (66 doctests)
• cargo +1.85.1 check --workspace --all-targets --locked
• cargo clippy --all -- -D warnings
• cargo llvm-cov --workspace --all-targets --lcov --output-path /private/tmp/svar-after.lcov (local line coverage: 84.63% to 89.24%)
• scripts/cargo_readme.sh core --check
• scripts/cargo_readme.sh cli --check
• pre-commit: typos, fmt, clippy, doc tests, unit tests, and README checks

[codecov]

Codecov Report

❌ Patch coverage is 95.83333% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.26%. Comparing base (1506578) to head (5ae6181).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
...ates/core/src/models/question/security_question.rs	93.75%	3 Missing ⚠️
crates/core/src/security_questions_sealed.rs	62.50%	3 Missing ⚠️
...models/answer/security_question_answer_and_salt.rs	95.65%	1 Missing ⚠️
...els/answer/security_questions_answers_and_salts.rs	97.56%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #3      +/-   ##
==========================================
+ Coverage   93.98%   94.26%   +0.28%     
==========================================
  Files          19       19              
  Lines         565      610      +45     
==========================================
+ Hits          531      575      +44     
- Misses         34       35       +1     

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