[cyclops] Absorb DescribeResultCache into GraphController

[dglazkov]

What

Replaces the standalone DescribeResultCache with signal-backed describe
entries owned directly by GraphController, injecting the describe capability
as a NodeDescriber function at the SCA Service→Controller boundary.

Why

DescribeResultCache was the last piece of derived state not owned by
GraphController. Absorbing it gives the controller full control over all
reactive state (nodes, graphs, tools, components, describe), clearing the way
for eliminating the event bridge in the next stage.

Changes

@breadboard-ai/types

• Remove InspectableDescriberResultCache and
  InspectableDescriberResultCacheEntry types
• Add NodeDescribeSnapshot type
• Replace MutableGraph.describe field with describeNode() method

visual-editor — New files

• node-describer.ts — NodeDescriber function type (SCA Service→Controller
  boundary)
• node-describe-entry.ts — Signal-backed NodeDescribeEntry class

visual-editor — Modified

• graph-controller.ts — Owns #describeEntries map, adds describeNode() and
  #refreshDescribers(), accepts NodeDescriber in initialize()
• initialize-editor.ts — Builds NodeDescriber closure from getHandler +
  sandbox
• node.ts — Uses describeNode() instead of describe.get()
• graph.ts — Import cleanup

visual-editor — Deleted

• describe-cache.ts — Functionality absorbed into GraphController

Tests & helpers

• _mutable-graph.ts — Updated mock with describeNode()
• graph-controller.test.ts — Added noopDescriber arg, replaced .describe
  assertions
• node-actions.test.ts — Added noopDescriber arg

Testing

• npm run build:tsc -w packages/visual-editor — compiles clean
• npm run test -w packages/visual-editor — all tests pass

[github-actions]

📊 Coverage Report

Metric	PR	Main	Delta
Lines	98.15%	98.38%	🔴 -0.23%
Functions	98.10%	98.23%	🔴 -0.13%
Branches	94.29%	94.37%	🔴 -0.08%

🔴 2 files with reduced line coverage

File	PR	Main	Delta
initialize-editor.ts	85.13%	100.00%	🔴 -14.87%
graph-controller.ts	96.45%	96.74%	🔴 -0.29%