Rule pack: portability/*

[mike-north]

Rule pack: portability/*

Labels: backlog → P1 when #61 merges (parallelizable with #62–#65 once the engine exists).
Blocked by #61.

Problem

Implement the cross-harness portability rule pack per docs/specs/lint-engine.md §3.5 — the
superposition-authoring payoff: warn when authored config won't survive translation to a
declared target.

Acceptance criteria

1. Rule: portability/untranslatable-feature, grounded in docs/specs/adapter-system.md §4.2.1
   assertability tables; fires only for targets declared in the plugin's envelope. Registered
   with meta per L-D4.
2. Tests derive expectations from the adapter-system tables and cite the table rows in test
   comments; negative tests confirm no diagnostics for fully-translatable config and for
   features on targets not in the envelope.
3. Per-rule docs stub.
4. pnpm run check + dogfood aipm validate green; changeset (minor, stay 0.x).