refactor: add registerSync() to PluginRegistry, eliminate registryInternals hack (#77)

[MarcosBrendonDePaula]

Summary

• Added registerSync(), refreshLoadOrder(), and getPluginsMap() public methods to PluginRegistry
• Removed the PluginRegistryInternals interface and all 12 as unknown as encapsulation violations from server.ts
• Added 20 unit tests covering sync registration, load order, and edge cases

Closes #77

What was the problem?

// server.ts — 12 occurrences of this pattern:
(this.pluginRegistry as unknown as PluginRegistryInternals).plugins.set(...)
(this.pluginRegistry as unknown as PluginRegistryInternals).dependencies.set(...)
(this.pluginRegistry as unknown as PluginRegistryInternals).updateLoadOrder()
(this.pluginRegistry as unknown as PluginRegistryInternals).loadOrder = ...

Breakable silently if private fields are renamed, contradicts the framework's type safety goals.

What changed?

• core/plugins/registry.ts: Added 3 typed public methods (registerSync, refreshLoadOrder, getPluginsMap)
• core/framework/server.ts: Replaced 12 private-field accesses with public API calls; removed PluginRegistryInternals interface and getter (-54 lines, +5 lines)
• tests/unit/core/plugins/registry-sync-api.test.ts: 20 tests covering dependency chains, diamond deps, priority ordering, circular detection, mixed scenarios

Test plan

• New test file passes (20/20 tests)
• Full test suite passes (588 passed, 0 failed)
• bun run dev starts without regressions

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