fix(macros): resolve Schematic paths through core or gotcha

[Kilerd]

#[derive(gotcha::Schematic)] currently fails in a downstream application unless it adds a direct gotcha_core dependency and imports the trait. Resolve generated paths centrally through a direct core dependency or gotcha's core re-export, and qualify the generated trait reference.

Schematic remains defined in gotcha_core. Libraries can implement or derive it using core alone, and their types satisfy the application's re-exported trait. The derive recognizes Cargo dependency renames and supports #[schematic(crate = "path::to::gotcha_core")] for custom facades. All schema generators and field metadata use the selected path; schema generation behavior is unchanged. Crate discovery adds a macro compilation dependency, with no new runtime work.

Also re-export gotcha's existing validator crate. The documented single-dependency validation form uses #[derive(gotcha::Validate)] with #[validate(crate = "gotcha::validator")]; the explicit upstream override is still required. Remove the direct core dev-dependency workaround and update the two compiler diagnostic snapshots whose displayed paths changed with the re-export.

Validation:

• Five standalone Cargo fixtures cover core-only and gotcha-only consumers, both dependency renames, and a custom facade with neither direct dependency. They verify cross-library trait compatibility, validation, and field metadata without importing Schematic. Dependency-tree checks reject web dependencies in core-only consumers. These checks now run in CI.
• Gotcha default/all-features tests and doctests, core no-default/all-features tests and doctests, and macro unit tests passed.
• Workspace Clippy with warnings denied, formatting, and documentation build with warnings denied passed.
• All 11 GitHub CI checks passed for 15bab8ef0b4fdd7dc53483b37d386794480da5e8.

The feature-documentation inconsistency reported in #87 was already corrected by #115. The unrelated standalone build failure when disabling gotcha's default http1 feature is tracked separately in #116.

Fixes #87.