docs: add product strategy roadmap

[CoderDeltaLAN]

Summary:

• Add a product strategy roadmap for the post-v0.1 direction.
• Define the governance wedge, competitive boundaries, no-goals, claims policy, and version roadmap.
• Record that strategy documentation comes before v0.2 implementation.

Scope:

• Documentation only.
• Adds docs/PRODUCT-STRATEGY.md.
• No runtime code changes.
• No README changes.
• No CI changes.
• No release, tag, asset, or GitHub metadata changes.

Validation:

• ./scripts/check.sh passed.
• Isolated wheel/sdist build passed.
• Wheel install smoke test passed.
• Claim scan passed with forbidden claims kept only as prohibited claims or negated wording.
• Release/tag v0.1.0 verified unchanged.

Risk:

• Low; strategic documentation only.

Rollback:

• Revert this PR if the roadmap is too broad or the product boundary should be refined before v0.2 planning.