design: T-30 hollow-alias structural gate (fact-density checker)

[briansrls]

Summary

Adds docs/design-t30-hollow-alias-gate.md — the structural gate design for T-30, the hard prerequisite that unblocks T-4 (per-language primitive fact-bundles).

The design ratifies the existing .dag authority (v4.lens.fact_density classifier + fact_density_hollow_alias_gate, wired through always_required_lenses / run_required_lens_gates_on_subtree in 00_compile.dag). Hollow carriers are structurally NoFact (zero named edges on a TypeNode): bare Atom/Instantiation aliases and positional-only Conj. Kernel-ambient atoms are exempt; subtree enforcement is via compile-lens fold, not inside the gate fn.

Defines what "generated" means (.dag lowered via bootstrap, executed by validate_then_compile), names the P5(b) dissolution path for v4_hollow_alias_gate.rs, and phases A–D for the follow-on build slice.

Design only — single file, no code or CI workflow changes.

Test plan

• Design doc reviewed against INVARIANTS.md P1, MODELING.md M1, docs/modeling-discipline.md Practice 8
• doc_refs green

[briansrls]

Addressing api-review on d942aac: squashed branch to a single design-doc commit (3cebea089b) on current main — drops the merge commit, interim ci.yml timeout bump/revert, and WIP commit from branch history. Diff vs main remains only docs/design-t30-hollow-alias-gate.md. PR body updated (removed stale v4_lens_gate waiver; #4719 root fix is on main).

— sent from loyal-moth-527