draft: feat(metrics): add PostgreSQL 19 multixact_size support using pg_get_multixact_stats()

Closes #84

Summary

Add native pg_get_multixact_stats() support for PostgreSQL 19+ to the multixact_size metric. This new SQL-level function is much faster than filesystem scanning (~0.1ms vs seconds).

Changes

• Added new PostgreSQL 19 version of multixact_size metric SQL
• Uses native pg_get_multixact_stats() when available
• Falls back to existing Aurora/RDS/filesystem methods for cloud providers that may not have the native function yet
• Calculates offsets_bytes from num_mxids (each SLRU segment is 256KB with 32768 8-byte entries)

pg_get_multixact_stats() returns

Column	Type	Description
num_mxids	integer	Total count of multixact IDs
num_members	bigint	Total number of member entries
members_size	bigint	Bytes occupied by members in pg_multixact/members/
oldest_multixact	xid	Oldest multixact ID still needed

Test plan

• Verify YAML syntax is valid (done locally)
• Test on PostgreSQL 19 dev build with native function
• Verify fallback works on PG18 and earlier

References

• PostgreSQL commit: https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=97b101776ce23dd6c4abbdae213806bc24ed6133