Slice: Author Jim lawpacks for CreateBuffer TextWindow and DeclareCheckpoint

[flyingrobots]

Slice

After #296 proves the first Jim-owned Edict operation, author the remaining
lawpacks required by the smallest active-observer loop: TextWindow,
CreateBuffer, and DeclareCheckpoint.

Parent: #295.

Ordering

1. Author TextWindow.edict first so Jim.edict can obtain a basis-bound
   reading before deriving a mutation intent.
2. Author CreateBuffer.edict for initial Jim-owned rope facts.
3. Author DeclareCheckpoint.edict while preserving the separation between a
   Jim checkpoint proposition and an Echo causal anchor.

Acceptance criteria

• Each operation/optic has checked-in Jim-owned Edict source and complete
  declared closure.
• Edict emits exact compiler-owned artifacts through its public build
  boundary and a structurally separate verifier accepts them.
• Echo generically interprets the exact packages without application
  callbacks, vocabulary, or reconstructed meaning.
• TextWindow returns a basis, aperture, budget, result posture, and support
  evidence suitable for Jim.edict decision-making.
• CreateBuffer produces exact Jim-owned initial rope facts and opaque Echo
  evidence.
• DeclareCheckpoint writes only the Jim checkpoint proposition and cannot
  mint a text head, rewrite, diff, text mutation, or Echo causal anchor.
• Every program survives restart through Echo-owned durable history.
• Missing package, retained evidence, budget, or basis returns a typed
  obstruction.
• Direct generated clients remain test-only harnesses or semantic-free
  transport stubs; no frontend selects these operations in final production.
• No operation falls back to the GraphQL/Wesley compatibility host.

Non-goals

• Hosting the Jim.edict state machine is a separate slice under Goalpost: Run Jim.edict as an Echo-realized active observer #295.
• No save/export, causal inverse, multi-range mutation, or ambient host
  capability.

[coderabbitai]

🔗 Related PRs

#9 - test(stack): add Stack Witness 0001 jedit transport witnesses [closed]
#23 - feat(echo): add Echo release gate witness [closed]
#27 - feat(echo): wire trusted runtime lifecycle into jedit [closed]
#280 - Add runtime-backed causal range evidence [merged]
#282 - Route checkpoint declarations through Echo [merged]

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