Add static dependency extraction from build files

[andrew]

Add bounded static dependency extraction for build files whose source syntax can contain executable or dynamic expressions.

• Meson: extract literal metadata from project() and literal dependency names from dependency() calls in meson.build.
• Nix: extract dependency declarations from common mkDerivation forms in default.nix.
• Bazel: extract bounded dependency declarations from BUILD files.
• Buck: extract bounded dependency declarations from BUCK files.

Parsing must not evaluate the file, load adjacent source, expand variables, call functions, or resolve imports. Dynamic expressions should be skipped without discarding independent literal declarations. Add parser registration, unit tests, fixtures, and README entries for each checkbox.