Make problem creation registry-driven and model-owned

[isPANN]

Summary

• replace the monolithic derived CreateArgs with a deferred, registry-driven Clap parser
• derive every selected variant’s flags and concrete numeric parsers from its typed construction contract
• move exceptional construction semantics into model-local CreateSpec + TryFrom implementations
• make CLI and MCP invoke the same registered constructor and emit canonical serialized model JSON
• make optional random generation a model-owned, per-variant capability
• keep the existing algebraic/formula/graph/misc/set taxonomy while declaring each category explicitly as typed schema metadata
• remove superseded model-name dispatch, derived-field tables, compatibility aliases, schema-semantic branches, and the enlarged-thread-stack workaround
• keep static shell completion generation shallow so it does not recursively expand every model’s dynamic flags

Resulting extension contract

An ordinary new model registers its schema and variants in the model module; CLI and MCP discover it without frontend changes. A model whose public construction input differs from its persisted representation defines a local typed CreateSpec and fallible conversion. Random generation is registered only for variants with a meaningful generator. Each schema explicitly selects one of the five established structural categories.

Unknown inputs, missing required inputs, invalid values, unsupported random generation, and missing registry metadata fail explicitly. There are no model-name fallbacks in CLI or MCP.

Stack

• Base: Exact symbolic growth, unified reduction size contracts, and deterministic solver backends #1083 (1075-growth-domain)
• This PR intentionally targets that branch and must be retargeted to main after Exact symbolic growth, unified reduction size contracts, and deterministic solver backends #1083 merges.

Known trade-offs

• pred create builds lightweight subcommand shells for registered variants, but expands dynamic flags only for the selected variant. Startup work therefore remains linear in the number of registered variants.
• Static shell completions include the stable command/problem layer but intentionally omit dynamically expanded per-model flags.

Verification

• cargo test --workspace --all-targets --features mcp
• cargo test -p problemreductions-cli --all-targets --features mcp
• cargo test -p problemreductions --doc
• cargo clippy --workspace --all-targets --features mcp -- -D warnings
• construction-contract, random-contract, bounded-stack, numeric-parser, category-filter, and graph-export focused tests
• manual pred list, unknown-category, SCS derived-field, and composite graph construction checks

Closes #1133
Closes #1135