[STUBGEN][RUST] Skip crate-provided types and add a custom constructor example

[Seven-Streams]

Summary

prefix: ffi could generate a non-generic Array that shadowed the crate's
Array<T>, causing E0107. Prefix expansion and --init now use
Generator.is_builtin to skip runtime-provided types. Rust shares _provider's
type-key rule; Python continues to use BUILTIN_TYPE_KEYS.

examples/rust_stubgen combines the demand-driven layout from #752 with a
hand-written IntPair::new(a, b) that derives kind from its operands.
custom-new renames the generated allocator to from_complete_fields, and
STUB_INIT OFF keeps regeneration in place. Linux CI builds and runs the example
and checks for stale generated bindings.

Testing

• Stubgen tests: 109 passed, including ffi.Array regression coverage.
• ruff, ty, and yamllint: passed.
• Example builds and runs; regeneration leaves bindings unchanged and --check passes.
• Verified duplicate-new errors, restoration of deleted blocks, and generation
  of blocks for new types.