feat(symbian): graduate the E7 target into platform contracts

[doodlewind]

Context

PR #176 deliberately keeps symbian-e7-dev as a private tool-owned target profile. Moving the complete resolver adapter into contracts/spec/ now would invert dependencies and expose an experimental target prematurely. When the port is ready for public support, only the target data belongs in the production registry; tool-specific admission wiring should remain outside contracts.

Acceptance criteria

• Define explicit graduation gates for device coverage, repeatable goldens, lifecycle, input, packaging, and support policy.
• Register the production target data in contracts/spec/platforms.ts with a stable target id and host ABI.
• Keep manifest resolution target-neutral and remove the private registry only after all consumers use the production contract.
• Update schemas, package metadata, documentation, and compatibility tests without target-specific admission exceptions.
• Preserve PSP, Vita, and macOS widget plan and golden behavior.