feature(op): E2E tests, full-signature callables, struct serialization (Phase 6) - #202

…n (Phase 6)

Phase 6 — E2E tests + two correctness fixes + phase documents:

Immediate bridge callable support:
- callNonVariadic detects starlark.Callable values targeting func-typed
  Go params and adapts them via buildCallableFunc (full-signature
  Go↔Starlark marshaling). Thread passed through from builtinFunc closure.
- Blank import of mem package in test runner for callable extractor
  registration (needed by planned mode ExtractCallable).

Full-signature callable matching (Issue #1):
- Removed Actor convenience function and +devlore:callable swallow=stack
  annotation. The callable must match the full Go Reducer signature
  (initial, resource, relativePath, stack).
- Removed arity truncation from buildCallableFunc — all Go args passed
  to Starlark.
- Updated starcode/provider.go Actor usage to full Reducer signature.
- All test callables updated from 3-param to 4-param.

Full-fidelity struct serialization (Issue #2):
- FormatLiteral now handles *starlarkstruct.Struct values by serializing
  as dict literals with sorted keys. Enables Resources captured in
  closures to be serialized with full fidelity.
- 3 new tests: Struct, NestedStruct, EmptyStruct.

E2E test scripts:
- test_walk_tree.star: immediate mode — walk temp dir, collect+sort paths
- test_walk_tree_planned.star: planned mode — plan.file.walk_tree
- test_walk_tree_gitignore.star: .gitignore filtering
- test_walk_tree_closure.star: def capturing closure variable (ext=".py")

Phase documents:
- Created docs/plans/mem-resource/phase-0.md through phase-7.md
- Updated master plan: removed stale arity truncation/swallow references,
  updated current state, Phase 6 marked done

…/devlore-cli into feature/mem-resource-phase-6