feat(strategies): Base.merge for StrategyRegistry with cross-registry validation

[ocots]

Summary

• Adds Base.merge(a::StrategyRegistry, b::StrategyRegistry...), running the equivalent of create_registry's cross-registry checks instead of a raw Dict merge, which silently let a colliding ID resolve to whichever entry won.
• Validated invariants:
  • every strategy id() is unique across all input registries;
  • a parameter id() shared by more than one input registry resolves to the same parameter type in each;
  • no id() is used as a strategy ID in one registry and a parameter ID in another.
• On the open design question the issue raised: when the same family is registered in more than one input, their strategy lists are unioned (still subject to the checks above), rather than rejected — this is what makes merge useful for a downstream package extending a family across two independently-built registries (e.g. a solve registry plus a flow registry), not just combining registries with disjoint families.
• Extends Base.merge rather than introducing an exported Strategies.merge, per the issue's naming note, avoiding the shadowing problem that made downstream packages move away from @reexport.

Fixes #517

Test plan

• New test/suite/strategies/test_registry_merge.jl (33 tests), organized as Unit / Integration / Contract / Error:
  • disjoint-family merge, 2- and 3-registry family unions of distinct strategies, the identity case
  • describe/show exercised through the merged registry, confirming a unioned family renders under a single header
  • a parameterized-strategy union and parameter id/type agreement across registries
  • every rejection path, including the key boundary: the same strategy id under a shared family across registries is still rejected, not silently merged
• suite/strategies (1013 tests) green
• Full suite green: 4956/4956

🤖 Generated with Claude Code