Resolve constant-reference dependencies and safely inferred import paths

[TristanKruse]

Autoloaded code such as Retrieval::Store.new and imports such as path = '../retrieval/store'; require_relative path previously contributed no architecture dependency. Both issue reproducers now produce the expected single forbidden file dependency without executing application code.

• Add opt-in CheckOptions.new(dependency_analysis: :imports_and_constants) with a bounded Prism definition/reference index. Import-only analysis remains the default. Lexical nesting, compact/root-qualified namespaces, namespace shadowing, and explicit superclass/mixin references are covered; ambiguous reopenings and unsupported lookup stay unresolved.
• Infer literal local strings across the four supported import forms, with conservative invalidation for reassignment, aliases, mutation/escape, closures, reflection, and unsupported control flow.
• In expanded mode, merge import and constant support into one edge while preserving names and source spans. JSON exports retain evidence; file/layer/slice/cycle/diagram rules and coupling metrics consume the same graph. Import-only edges keep empty evidence for compatibility with existing edge-value comparisons, including the RAG fixture. Analysis mode participates in cache identity.
• Document the bounded contract and Prism/RubyDEX design choice. Extend the benchmark and CI with constant-index performance and installed-gem regression checks.

This PR is based on fix/definition-of-done-blockers (#51), keeping the extraction diff separate from its documentation/quality prerequisites. Review and merge #51 first, then retarget this PR to main if GitHub has not done so automatically. The overlapping changelog, CheckOptions, and metric-documentation edits have already been resolved together. Both PRs include the merged documentation and debug-inspection changes (#52 and #53); no gem release is triggered.

The combined branch passes 499 examples with zero failures on Ruby 4.0.6 / Windows x64, with 98.98% line and 90.94% branch coverage. RuboCop checks 248 files with no offenses, 176 generated documentation pages and their links validate, and the strict 0.0.2 gem build plus installed-gem reproducers pass. One existing symlink example is skipped because this Windows account cannot create symlinks. The GitHub matrix additionally verifies Ruby 3.3/3.4 on Ubuntu and 4.0 on Windows, plus the RAG fixture and installed-gem reproducers.

Three fresh-process 1,000-file benchmark runs per mode: median cold graph build 1.706 s import-only vs 1.533 s expanded; median constant indexing 32.0 ms. Filesystem/run-order noise prevents interpreting the total-time difference as a speedup. All warm checks reuse the graph. Memory samples and their GC-related limits are recorded in benchmark/README.md.

Closes #54
Closes #55