Integrate thiserror and anyhow for structured error handling

[Copilot]

Replaces ad-hoc string-based error handling with typed errors using thiserror for library code and anyhow for CLI context.

Changes

Core Error Types (ktio::error)

Defined 5 error variants covering the entire error surface:

• IoError: File operations (auto-converts from std::io::Error)
• ParseError: Sequence format/data parsing
• InvalidInputError: Invalid nucleotides or sequences
• ConfigError: Parameter validation
• SystemError: Memory mapping and resource allocation

Library Updates

• ktio: seq.rs, mmap.rs return ktio::Result<T>
• composition: cgr.rs, oligo.rs, oligocgr.rs use typed errors
• coverage: Updated to use ktio::Result
• counter, misc: Dependencies added for future migration

CLI Integration

CLI functions now return anyhow::Result with context chains:

// Before
if let Err(e) = com.vectorise() {
    eprintln!("Error: {}", e);
}

// After
com.vectorise()
    .context("Failed to generate oligonucleotide vectors")?;

Error output includes context and cause chains:

Error: Failed to generate oligonucleotide vectors

Caused by:
    0: IO error: No such file or directory (os error 2)
    1: No such file or directory (os error 2)

Documentation

Added ERROR_HANDLING.md with usage patterns and migration guide.

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All 45 tests pass. No behavioral changes to existing functionality.

Original prompt

Outline a plan to integrate thiserror and anyhow for more consistent error handling. decide on 5 error types at most.

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[codecov-commenter]

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Codecov Report

❌ Patch coverage is 88.23529% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.19%. Comparing base (78f19f2) to head (b5d853f).

Files with missing lines	Patch %	Lines
composition/src/cgr.rs	75.00%	2 Missing ⚠️
composition/src/oligocgr.rs	75.00%	2 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##             main       #5      +/-   ##
==========================================
- Coverage   94.44%   94.19%   -0.25%     
==========================================
  Files          13       13              
  Lines         918      913       -5     
==========================================
- Hits          867      860       -7     
- Misses         51       53       +2     

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