perf: (PR 3/4) add secret-safe baseline invocation

[Luodian]

This defines the secret-safe legacy invocation union. It depends on canonical JSON and fail-closed redaction from the two lower layers.

Stack

• 1/4 perf: (PR 1/4) add RFC 8785 JSON primitives #1452 - RFC 8785 JSON primitives
• 2/4 perf: (PR 2/4) add fail-closed legacy redaction #1453 - fail-closed redaction
• 3/4 perf: (PR 3/4) add secret-safe baseline invocation #1454 (this PR) - secret-safe invocation
• 4/4 perf: (PR 4/4) capture baseline provenance #1455 - baseline provenance
• Next stack: perf: (PR 1/4) add baseline phase recorder #1456 starts performance measurement

Summary

• Define normalized and digest-only legacy invocation representations.
• Canonicalize secret-safe arguments under a domain-separated SHA-256 digest.
• Validate the closed invocation union before it enters a baseline record.

In scope

• Private legacy invocation construction, validation, and union-level security regressions.

Out of scope

• Source provenance, phase timing, or evaluator integration.

Validation

• Invocation contracts | sample size: N=51 | key metrics: 51 passed | result: pass
• Cumulative JSON, redaction, and invocation suite | sample size: N=153 | key metrics: 153 passed | result: pass
• Final stack gates | sample size: N=203 / 380 / 677 | key metrics: 203 performance and 380 hermetic passed; 677 collected | result: pass

Risk / Compatibility

• Private and unused by current evaluator paths; raw credentials never enter either union arm.
• GitHub stack #1469, layer 3/4; land before perf: (PR 4/4) capture baseline provenance #1455.

Type of Change

• Bug fix (non-breaking change)
• New feature
• New benchmark/task
• New model integration
• Breaking change
• Documentation update
• Refactoring (no functional changes)