Generic alter <type> X { set / insert / replace / drop } with per-doctype target resolvers

[ako]

Plan item 4.1 in PROPOSAL_mdl_beta_syntax_freeze.md §8.3. Implements ADR-0012, decision 2. On the beta gate.

Goal

One alter grammar for every document type, in place of about 15 per-type forms:

alter <type> Module.Name {
  set ( Key: value ) on <target>;
  insert before|after|into <target> { <fragment> }
  replace <target> with { <fragment> }
  drop <target>;
}

Fragments use exactly the create syntax.

Scope

• The grammar rule, with a per-document-type target resolver interface in mdl/backend.
• Port alter page, alter snippet and alter layout (pagemutator) and alter workflow (wfmutator) onto it. Their current forms become registered aliases (see the deprecation registry issue).
• Both backends: modelsdk and --mcp, or an explicit "not supported by this backend" error.

Done when

The existing alter tests pass through the new path, and the old forms still parse, with deprecation warnings.