feat: make problem construction model-owned

[isPANN]

Closes #1133

Stacked on #1132. Review this PR against codex/dynamic-create-schema.

What changed

• Added a model-owned CreateSpec contract with derived static input metadata, reusable codecs, strict unknown/missing-input errors, and type-erased construction callbacks on VariantEntry.
• Kept ordinary models zero-boilerplate while moving exceptional construction, derived fields, defaults, renames, composite graph inputs, and invariant checks beside their model types.
• Made CLI and MCP resolve the concrete variant, normalize values from the registered contract, call the same constructor, and serialize the resulting typed problem.
• Deleted the central schema semantics module and superseded model-name construction branches.
• Made Clap numeric parsers follow the selected variant concrete type, including u64::MAX and arbitrary-precision integer inputs.
• Updated add-model documentation and added catalog, CLI, MCP, model, and numeric-boundary regression coverage.

Random instance generation remains a separate existing capability; this PR replaces the non-random construction path.

Verification

• cargo test --workspace --all-features
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo fmt --all -- --check
• git diff --check
• Manual SCS positive/derived-field negative checks
• Manual ThreePartition u64::MAX construction check