feat(build system): consolidate YAML/SQL serialization

[hussainsultan]

This PR adds roundtrip translation of expr to and from YAML and defines what the build artifacts should look like.

• Build Outputs & Config:
  • Added metadata.json and BuildConfig support (with YAML alias/anchor disabling)
  • Updated BuildManager to generate SQL plans and support proper SQL formatting
• YAML Serialization:
  • kwargs_tuple in profile so it’s written as a key/value pair in YAML
  • Split deferred reads and SQL into dedicated YAML files (e.g. deferred_reads.yaml, sql.yaml)
  • Consolidated redundant YAML schemas in definitions field and added a schema_ref
• CachedNode:
  Note: CachedNode is not written in sql.yaml or deferred_reads.yaml
  • only caching with SourceStorage is supported
  • Improved error handling and adjusted tests (e.g., unmarking xfail tests for deferred reads)

XFail Cases:
• tests/test_compiler.py::test_ibis_compiler
• tests/test_letsql_ops.py::{test_memtable, test_memtable_cache}
Reason: MemTable is not serializable
• tests/test_udf.py::test_built_in_udf
Reason: UDFs do not have the same memory address when pickled/unpickled

Build Directory Structure:
├── c0907dab80b0
│  ├── c0907dab80b0.sql
│  ├── deferred_reads.yaml
│  ├── expr.yaml
│  ├── metadata.json
│  ├── profiles.yaml
│  └── sql.yaml

Handle in follow-up PRs:

• Add _register.py for code-based UDF serialization
• Write cache data to deferred_reads?
• Support pyarrow UDF

[codspeed-hq]

CodSpeed Performance Report

Merging #525 will not alter performance

_{Comparing feat/yaml-compiler (6972299) with main (cf5dc1a)}

Summary

✅ 2 untouched benchmarks

[codecov]

Codecov Report

Attention: Patch coverage is 53.57143% with 39 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
python/letsql/vendor/ibis/backends/__init__.py	51.85%	39 Missing ⚠️

Files with missing lines	Coverage Δ
python/letsql/config.py	69.51% <100.00%> (-30.49%)	⬇️
python/letsql/vendor/ibis/backends/__init__.py	48.87% <51.85%> (-20.44%)	⬇️

... and 189 files with indirect coverage changes

[ghoersti]

@hussainsultan This would break with new profiles implementation