docs: make SMT reconstruction specification independent

[johndauphine]

What changed

• redefine SMT as a complete standalone schema-migration and deterministic DDL project
• remove the normative surrounding-data-migration-tool boundary and host scheduling requirements
• keep optional use by unrelated schema migration systems explicitly non-normative
• make project independence a definition-of-done criterion

Why

SMT and DMT are independent projects. The reconstruction specification must not require a shared architecture, dependency, implementation, release, configuration, artifact, or lifecycle.

Impact

Reimplementations now treat SMT as the complete product described by the specification. The deterministic rendering, CLI, public API, artifact, and acceptance contracts are otherwise unchanged.

Verification

• go test ./...
• git diff --check
• verified RECREATE_SMT.md is the only changed file
• verified the document contains no DMT, data-migration-host, or surrounding-tool coupling references
• checked module, Go toolchain, SMT version, renderer version, and section numbering against current main