Self-packaging #4: embed:// DuckDB FileSystem (with Glob + SeekPosition)

[jrosskopf]

Part of epic #40. Depends on #3 (ArchiveContents shared state).

Goal

Let SQL templates reference bundled files directly:
read_csv('embed://data/cities.csv'). This bypasses IFileProvider
and goes through DuckDB's VirtualFileSystem, so we need a custom
duckdb::FileSystem subclass registered with the DuckDB instance
flapi already owns.

Scope

• New src/duckdb_embed_fs.{hpp,cpp} subclassing duckdb::FileSystem.
• Constructor takes the same std::shared_ptr<ArchiveContents> as
  the embedded file provider (Ability to pass default values to query parameters #3) — one decompressed map, two readers.
• Override:
  • OpenFile — return a custom FileHandle over an in-memory blob.
  • Read (both overloads), Seek, GetFileSize.
  • Glob — read_csv() expands paths before opening; base
    throws "not implemented" (spike caught this at runtime).
  • SeekPosition — same reason.
• CanHandleFile checks for embed:// scheme prefix.
• Register on the DuckDB instance once at startup
  (src/main.cpp, after the database manager exists) — only when a
  bundle is present.

Red tests (test/cpp/duckdb_embed_fs_test.cpp)

1. Proof-of-life (spike behaviour Feature Request: Support Apache Arrow IPC Streaming over HTTP #9):
   • In-process DuckDB instance.
   • Register EmbeddedFileSystem over a fixture archive containing
     data/cities.csv.
   • Execute SELECT * FROM read_csv('embed://data/cities.csv').
   • Assert the expected rows come back.
2. Glob('embed://data/*.csv') returns the expected entries
   (catches read_csv glob expansion).
3. Mixed read: a template that joins embed:// against a regular
   parquet path on disk works (no interference with default
   filesystem).

Green criteria

• All red tests pass.
• DuckDB tests in the existing suite still pass.
• make integration-test still green.

Files

• New: src/duckdb_embed_fs.{hpp,cpp}, test/cpp/duckdb_embed_fs_test.cpp
• Modified: src/main.cpp (register on DB instance when bundle exists)

Notes

• DuckDBVFSProvider already touches DuckDB's internal FileSystem
  API (src/vfs_adapter.cpp:218-397) and has TODOs about that being
  unstable. The embed FS uses the same surface — we accept the same
  trade-off, and the round-trip integration test (Flapi exe is not working for non docker local testing #6) catches drift
  on every CI run.