Rename 'sandbox' vocabulary to 'medium' throughout codebase

[deepfates]

Problem

The spec vocabulary says medium, not "sandbox." The codebase uses "sandbox" in ~50 places:

• GateDocs.sandbox_name field (should be medium_name)
• SandboxContext class in fs gates (should be WorkspaceContext)
• getJsMediumSandbox() export (should be getJsMediumContext())
• formatSandboxMetadata() in js.ts (should be formatMediumMetadata())
• Internal let sandbox: JsAsyncContext variable in js.ts (should be ctx)
• Entity-facing string "### SANDBOX PHYSICS (QuickJS)" (should be "MEDIUM PHYSICS")
• Local vars, test descriptions, eval harness labels across ~15 test files

NOT in scope: Chromium flags like --no-sandbox in browser/context.ts — those are external API.

Scope

This is a mechanical rename touching ~25 files. No behavioral changes. Full inventory in the plan file at .claude/plans/wondrous-plotting-tarjan.md.

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