Add Tables.jl result contract

[cvigilv]

Closes #26.

Summary

• add the exported, immutable KeggTable result type and its Tables.jl column-access interface
• standardize tabular responses on named String columns, including known schemas for empty and batched results
• report conversion and link columns as source_id then target_id, matching KEGG response order
• keep .data and .colnames as deprecated compatibility properties
• document direct DataFrame(result) conversion and named-column access

Tests

• julia +1.10 --project -e 'using Pkg; Pkg.test()'
• julia +release --project -e 'using Pkg; Pkg.test()'
• julia +release --project=docs docs/make.jl

[codecov]

Codecov Report

❌ Patch coverage is 96.61017% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/List.jl	90.00%	1 Missing ⚠️
src/Structures.jl	95.45%	1 Missing ⚠️

Files with missing lines	Coverage Δ
src/Conv.jl	100.00% <100.00%> (ø)
src/Ddi.jl	100.00% <ø> (ø)
src/Find.jl	90.00% <ø> (ø)
src/KEGGAPI.jl	100.00% <ø> (ø)
src/Link.jl	100.00% <100.00%> (+3.84%)	⬆️
src/Parsers.jl	100.00% <100.00%> (+41.79%)	⬆️
src/List.jl	78.12% <90.00%> (+29.64%)	⬆️
src/Structures.jl	95.65% <95.45%> (+28.98%)	⬆️

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