docs: add Product Spine schema contract

[yangfei222666-9]

Summary

Adds the Product Spine schema contract and updates the Product Spine verifier plan reference.

Scope

• docs/PRODUCT_SPINE_SCHEMA_v0.1.md
• docs/PRODUCT_SPINE_VERIFIER_PLAN_v0.1.md

Out of scope

• runtime Product Spine implementation
• provider calls
• broker/trading systems
• judgment / paper-buy / promote
• deployment
• remaining Win11/T7 rearchitecture inventory files

Safety

• draft PR only
• no merge
• no auto-merge
• no branch deletion
• local dirty/untracked files outside this scope are not included