Adds the host-side bridge between the plugin ABI and the document
format registry so plugin-provided parsers and emitters plug into
DocumentFormatRegistry the same way the in-tree adapters do. A plugin
that registers a parser through this surface ends up as a regular
adapter consumers can look up via registry.adapter(for:) — no plugin-
specific branch in the consumer.

The plugin-side invocation (how a plugin's invoke pointer gets wired
back into the shim adapter) is structured around a PluginDocumentInvoker
protocol so the host-to-plugin callback is a single seam. This PR
wires the Swift side end-to-end and tests it with a fake invoker;
the PluginManager plumbing that threads each plugin's real invoke
pointer into PluginDocumentInvoker lands with a follow-up since it
needs access to PluginManager internals.

- osaurus_plugin.h: adds osr_register_parser_fn / register_emitter_fn /
  unregister_format_fn signatures and the trailing struct fields,
  with full request/response JSON contract documented inline.
  Trailing fields — older plugins compiled against the v2 layout
  pre-this-PR keep loading because the host allocates the struct and
  zero-inits the new tail.
- PluginBackedDocumentAdapter.swift: Swift shims implementing
  DocumentFormatAdapter and DocumentFormatEmitter by forwarding to a
  plugin via PluginDocumentInvoker.invoke(type:id:payload:). Surfaces
  only the textFallback representation today; richer representations
  (Workbook, PDFDocumentRepresentation) come with a response-schema
  extension once a first plugin needs them.
- PluginDocumentRegistry.swift: owns format_id -> plugin_id
  ownership so one plugin can't unregister another's format (or
  overwrite an in-tree built-in). Returns JSON envelopes matching the
  C-header contract.
- Tests: 8 scenarios covering happy-path registration, adapter →
  plugin invocation threading, plugin error propagation, emitter
  routing, another-plugin-cannot-overwrite, reject-unregister-by-
  other, unregisterAll teardown on plugin unload, and malformed-JSON
  rejection.