docs: add v0.1 build plan

[CoderDeltaLAN]

Summary:

• Add docs/BUILD-PLAN.md with the v0.1 construction plan.

Scope:

• Documentation only.
• One new file.
• No README finalization.
• No CI changes.
• No code changes.

Validation:

• Local checks passed.
• Pre-push strong check passed.
• Diff reviewed.
• No secrets added.

Risk:

• Low. Planning document only.

Rollback:

• Revert the PR commit if needed.