pass: give transform passes a sanctioned way to replace and remove types #966

Problem

architecture.md §2.2 designs five transform passes: link, dedup, filter, version-slice and overlay. emitter-design §2.4 has the engine run version-slice before any emitter. At c387e25c none of them can be written as designed.

• The registry rule refuses it. A pass that replaces or removes a type writes the type registry. TestRegistryWrites_StayInsideTheFramework refuses that outside compilers/compile and ir.
• The sanctioned writer is out of reach. That writer is compile.Types, which pass/ may not import: archtest allows it ir only.
• There is no copy helper. ir has no helper to copy a TypeDef, only ir.NewTypeDef. A throwaway pass that trims whitespace from Docs.Summary needed reflect to rebuild each type.
• The rule is easy to dodge. It is syntactic: out.Types[id] = … is flagged, while maps.Clone into a local variable and a write through that variable is not.

The engine side is fixed as well:

• the pass list is defaultPasses;
• engine.NewWith takes compilers only, so an embedder cannot add a pass;
• two tests pin the exact list (engine/passes_test.go, cmd/morphic/validate_test.go);
• the typed per-pass settings and the passes that cannot be disabled, both decided in §2.2, have no mechanism yet.

Direction

A proposal:

• An ir-level copy-on-write editor. For example, ir.Edit(doc) returns a builder whose ReplaceType, RemoveType and Done produce a new Document sharing what was not touched, and keep invariant 3's one node per ID.
• ir already owns the registry, so the editor needs no new owner. The archtest rule then names the editor as the way in for passes.
• engine.NewWith accepts passes for embedders, and the pinned-list tests read the list rather than repeating it.
• The required-pass mechanism lands with the first pass that needs it.

Done when a filter pass that removes an unused type is written without reflect and without tripping the registry rule.

Out of scope

Any particular transform. Usage may not need a pass at all (#950).

Related: #950, #689

Priority: P3, rule 3 (off the golden spec: no transform changes what it compiles to today; a later emitter needs version-slice and filter). A pass writing out.Types[id] → TestRegistryWrites_StayInsideTheFramework fails (morphic @c387e25c).