feat(puffin): add format constants, utilities, and JSON serialization

[zhaoxuan1994]

Second PR in the Puffin series (following #588).

puffin_format.h/.cc: format constants, footer flag operations, little-endian I/O, compress/decompress dispatch (LZ4/Zstd stubbed as NotSupported)
puffin_json_internal.h/.cc: JSON serialization for BlobMetadata and FileMetadata

[Copilot]

Pull request overview

Adds core Puffin file-format helpers and JSON (de)serialization support for Puffin metadata, plus unit tests and build-system wiring to compile/run the new components.

Changes:

• Introduce puffin_format constants/utilities (footer flags, little-endian I/O, compression dispatch stubs).
• Add puffin_json_internal JSON serialization/deserialization for BlobMetadata and FileMetadata.
• Add puffin_test and register it in both CMake and Meson test targets.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/iceberg/puffin/puffin_format.h	Declares Puffin format constants and helper APIs (flags, endian I/O, compress/decompress).
src/iceberg/puffin/puffin_format.cc	Implements flags/endian helpers and compression dispatch stubs.
src/iceberg/puffin/puffin_json_internal.h	Declares JSON (de)serialization APIs for Puffin metadata.
src/iceberg/puffin/puffin_json_internal.cc	Implements JSON (de)serialization and string parsing for FileMetadata.
src/iceberg/test/puffin_test.cc	Adds unit tests for endian/flags/compression stubs and JSON round-trips.
src/iceberg/test/CMakeLists.txt	Registers puffin_test in CMake-based test build.
src/iceberg/test/meson.build	Registers puffin_test in Meson-based test build.
src/iceberg/meson.build	Adds new Puffin .cc sources to the main Meson library build.
src/iceberg/CMakeLists.txt	Adds new Puffin .cc sources to the main CMake library build.
src/iceberg/puffin/meson.build	Updates Puffin header installation list.

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[Copilot]

PuffinFormat::kFooterCompressionCodec is set to kLz4, but Compress/Decompress currently return NotSupported for LZ4. This constant is likely to be used as the default when writing/reading footer payloads, and will cause immediate failures once that code lands. Consider defaulting to kNone until LZ4 support is implemented, or clearly documenting that the default codec is not yet supported and must not be used for writing.

Suggested change

	PuffinCompressionCodec::kLz4;
	PuffinCompressionCodec::kNone;