docs(specs): diagram-traversal algebra derivation (expand/restrict) in diagram.md

[dimitri-yatsenko]

Consolidates the full diagram-traversal derivation into the datajoint-docs specs, where the normative specs live — relocating it from datajoint-python's DESIGN-expand-restrict.md (PR #1524, now closed).

What

Adds a "Traversal algebra" design rationale to reference/specs/diagram.md deriving the traversal operations from first principles:

• one primitive — propagating a restriction across a foreign key is itself a restriction (&), either direction;
• R1 (edge rule) — restrict the neighbor by the projected/renamed restricted table; identity fast-path when the FK is the whole PK, unrenamed;
• R2 (group rule) — a restriction touching any part lifts to the master and re-expands to all its parts;
• two operations — additive expand(direction) (of which cascade = down, trace = up) and subtractive restrict;
• renamed FKs — the three seed-restriction kinds (materialized / subquery / string) and the relabel fast-path;
• master↔part — R2 via relabel-drop + re-expansion, and why a key reaching a part via its master carries no part-specific constraint.

cascade.md and trace.md now point to this rationale as the downstream/upstream cases, keeping only their operation-specific content.

Notes

• Single source of truth: the derivation is in diagram.md; the other specs reference it rather than re-deriving.
• Terminology/notation follow the house style (restriction/anti-restriction, DataJoint operators, "diagram" not ERD).
• Supersedes the old-framing draft docs: diagram operations — fold cascade/trace/restrict derivation into the specs (WIP) #234 (which predates the expand/restrict model).
• Further de-duplication of cascade.md/trace.md's older per-rule enumerations (F1–3/U1–3) and the DiGraph→MultiDiGraph wording is a follow-up reshape.

[dimitri-yatsenko]

Revised the derivation to expand-only: dropped restrict and the additive/subtractive framing. The model is now one operation — expand(seed, direction in {up,down,both}) (cascade = down, trace = up, export = both) — over R1 (edge) + R2 (group). Multi-condition filtering reduces to seeding a common descendant or a union of expands, so it's a UI-layer composition, not a core primitive. (Note: the Operational Methods section still documents the shipped Diagram.restrict(); it's subsumed by expand in the model and would be removed when the redesign lands.)