fix: replace old storage format with the 1.5.2 to enable advanced compression methods

[drrtuy]

Pin DuckDB storage format to v1.5.2 to enable advanced compression

What

Newly created DuckDB databases used the conservative default storage
format, which gates out modern column compression (e.g. DICT_FSST) and
forces the legacy separate Dictionary/FSST encodings. This inflated the
on-disk database considerably. Pinning the storage version to v1.5.2
lets DuckDB pick the compact compression schemes.

Key changes

• runtime/duckdb_manager.cc: set config.options.serialization_compatibility to SerializationCompatibility::FromString("v1.5.2") in DuckdbManager::Initialize().
• Rename runtime/fiber_context.c → fiber_context.cpp and update runtime/CMakeLists.txt accordingly.

How to test

Create a fresh database and load ClickBench data, then:
SELECT run_in_duckdb('SELECT column_name, compression FROM pragma_storage_info(''test.hits'') GROUP BY 1,2');
Expect DICT_FSST on string columns and a smaller PRAGMA database_size.
Note: the storage version applies only to newly created files — existing DBs must be reloaded.

[gemini-code-assist]

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