Harden model-spec dependency option cross-checks

[lapy]

Summary

• Require dependencies[].option to exist under options.<scope>
• Require required_when[].option_key to refer to a declared option in that condition scope (public key form)
• Document OR semantics for multiple required_when rows
• Align the toy Qwen3 ASR demo options with the dependency rows
• Rebased onto latest main

Follow-up from the discussion on #89. Keeps top-level dependencies separate from options; this only hardens the schema gate.

Test plan

• model_spec_system_test passes locally (covers undeclared dependencies[].option / required_when.option_key)
• Spot-check: all 42 in-tree model_specs/*.json satisfy the new dependency/option cross-checks
• Existing Linux/macOS/Windows/Nix CI builds (no workflow changes in this PR — Linux CI does not currently run unit tests)

[0xShug0]

@lapy Is this PR ready for review? The test plan lists CI unit tests for the model-spec schema, but that item still appears incomplete.

[lapy]

Yes — ready for review now.

Updates since your comment:

• Rebased onto latest main (resolved the docs conflict)
• Wired model_spec_system_test into Linux CI (ENGINE_BUILD_TESTS=ON, CPU matrix)
• Confirmed the unit test passes locally, and all 42 in-tree model_specs/*.json satisfy the new dependency/option cross-checks

Test-plan checkboxes are updated.

[lapy]

CI came back cancelled (no runner / empty steps) after I added a Linux workflow change to run model_spec_system_test. Re-run also refused with “workflow file may be broken” — looks like the fork PR couldn’t execute the modified workflow.

I’ve reverted the .github/workflows change and pushed again so CI uses the existing build workflows only. Schema unit coverage remains in tests/unittests/test_model_spec_system.cpp and passes locally; happy to land a separate follow-up for CI unit tests from a branch with write access if you want that wired in.

[0xShug0]

@lapy Merged. Thanks!