[DOCS] Clarify predefined-size example and extend guidance guard

[neusse]

Summary

Follow-up to #357 and PR #366.

PR #366 correctly replaces fixed trace, via, and impedance lookup guidance with project-specific calculations, stackup evidence, fabrication constraints, netclass readback, and DRC acceptance. One example in crates/konnect/assets/skills/kicad-pcb/SKILL.md still uses literal values:

set_predefined_sizes(board, track_widths=[0.2, 0.5, 0.8],
    via_dimensions=[{diameter:0.6, drill:0.3}, {diameter:0.8, drill:0.4}])

The surrounding section explains that predefined sizes are a palette rather than DRC limits, but it does not explicitly say these numbers are illustrative. An agent could therefore reuse them as recommended defaults instead of substituting values from the accepted project sizing record.

The new manufacturing_guidance_is_contract_bound_and_fail_closed asset test checks the manufacturing and reference files, but it does not inspect this PCB skill example. The wording gap can therefore return without failing the guard.

Proposed focused change

• Label the set_predefined_sizes values as syntax-only examples, not engineering recommendations.
• Require callers to substitute values from the accepted project netclass/sizing record and current fabrication contract.
• Extend the asset guard to inspect kicad-pcb/SKILL.md for that authority statement.
• Temporarily remove or alter the authority statement and prove the regression test fails, per GOVERNANCE.md.

Acceptance criteria

• The predefined-size example cannot reasonably be read as universal trace/via advice.
• The skill explicitly binds the values to the accepted project sizing record.
• The asset test covers the PCB skill and fails when that binding is removed.
• No MCP schema, Rust runtime behavior, public API, or file format changes.
• The full repository gate passes.

Scope and priority

This is a small, guidance-only follow-up. It does not block the substantive manufacturing evidence corrections in PR #366.

[scorp508]

I am taking this issue. I will keep the change to the proposed guidance-only scope: mark the predefined-size values as syntax-only, bind replacements to the accepted project sizing record and current fabrication contract, extend the asset guard, and demonstrate that the guard fails when the authority statement is removed. I cannot self-assign through GitHub because my account is not currently assignable in this repository; please add the claimed label and assignment if needed.

[neusse]

Implemented by #414 and merged as e1bef202d0da6a93cde1d81d0e8eb140e6143829.

Acceptance evidence:

• The predefined-size example is explicitly syntax-only rather than universal engineering advice.
• Values are bound to the accepted project sizing record and current fabrication contract.
• The asset guard covers the PCB skill wording; the PR records the required negative-control mutation test.
• Scope remained guidance/tests only with no runtime, schema, public API, or file-format change.
• All 10 required GitHub checks passed on the exact reviewed head ccc418dfe051cd010927a1257805a13e635234c9.
• The complete post-merge gate passed: format, Clippy with all targets and warnings denied, workspace library/test targets, and documentation tests.