Add tools/export_ddl.py — multi-table DDL with foreign keys

[cloud3000]

Summary

Graduates find.py's --ddl into a full tools/export_ddl.py: export a
multi-table relational schema (with real foreign keys) for any set of business
terms.

How it works

• Reuses find.resolve — a term (category/entity/alias) selects items → one
  CREATE TABLE per entity.
• Foreign keys derived from the dictionary's relationship hints:
  • links to X (ERPNext) / references X (Odoo/Tryton) → single FK
  • multi-target FHIR references X, Y, Z → polymorphic, kept as a plain column
  • collection of X → reverse relation, listed as a comment (not a column)
• Surrogate-key modeling: a FK column becomes INTEGER referencing
  <target>_id, so the schema is internally consistent. FKs are emitted only
  when the target table is part of the same export (or a self-reference);
  otherwise the column is kept with an "external ref" comment.
• Dependency-ordered (topological sort) output so it loads under strict
  dialects.
• --dialect sqlite|postgres|mysql; --out FILE.

Verified

• work_order (single entity) and manufacturing (37 tables, 59 FKs) both
  validate by loading in SQLite.
• Cross-entity FKs resolve, e.g. within manufacturing:
  FOREIGN KEY (bom_no) REFERENCES bom(bom_id), self-ref on amended_from,
  FK to production_plan.
• postgres/mysql render correct types (SERIAL/TIMESTAMP, AUTO_INCREMENT/TINYINT).
  (SQLite is self-validated; pg/mysql are rendered — no server to load-test here.)

CHANGELOG [Unreleased] + README pointer added; ci_check green.

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