Resolve struct field types and byte offsets in the parsed IDL

[estebanzimanyi]

The catalog derives its struct layouts from the installed MEOS headers, giving FFI consumers (#[repr(C)] structs, cgo/cffi field access) accurate field types and byte offsets.

Where the layout comes from. The install step writes the generated, self-contained headers: meos_export.h splices postgres_ext_defs.in.h in place of the source tree's #include <postgres.h>. Parsing those headers, libclang resolves each field to its real C type and byte offset instead of degrading it to int at offset -1.

provision-meos. On the build-libmeos: true path the action builds libmeos and stages the MEOS headers into a dedicated prefix, then derives meos-idl.json from that prefix. A shared prefix such as /usr/local/include carries unrelated system headers (OpenSSL, PostgreSQL) that the header glob would pull into the catalog; the dedicated prefix holds only MEOS headers. The catalog-only path (build-libmeos: false) keeps the source-tree parse.

Parser. _clang_extra_args adds the compiler's own system include search path (from clang -E -v) so <stdbool.h> / <stddef.h> / size_t resolve — the pip libclang wheel ships no configured search path. size_t is force-included because meos.h uses it without an explicit #include. extract_struct runs field types through _c_spelling, normalising clang's _Bool keyword to the catalog's bool spelling.

Span: spantype:uint8@0, basetype:uint8@8, lower_inc:bool@16, upper_inc:bool@24,
      padding:char[4]@32, lower:Datum@64, upper:Datum@128

CI. The tests workflow builds all-families libmeos through the action, derives the catalog from the staged headers, and runs the suite. tests/test_struct_layout.py asserts the Span / STBox / TInstant layouts, that no field carries the _Bool keyword, and that every CORE-family struct field has a resolved offset.