docs(cayenne): document cayenne_metastore_* runtime.params tuning knobs

[claudespice]

Summary

The Cayenne runtime.params reference omits the entire cayenne_metastore_* sub-family — SQLite-metastore tuning knobs that the runtime reads from runtime.params at startup (crates/runtime/src/builder.rs) but that appear nowhere in the docs. This documents them, scoped per version (the family and its defaults differ across releases; verified against each tag).

These apply to the default sqlite metastore backend (already documented via cayenne_metastore); the turso backend uses MVCC and ignores them.

vNext + version-2.1.x (identical — 6 params):

Param	Default
cayenne_metastore_cache_mb	256
cayenne_metastore_mmap_mb	1024 (1 GiB)
cayenne_metastore_busy_timeout_ms	30000
cayenne_metastore_wal_autocheckpoint_pages	0 (inline auto-checkpoint disabled; WAL drained by background checkpoint)
cayenne_metastore_wal_truncate_threshold_mb	160
cayenne_metastore_auto_vacuum	none (none|incremental|full)

version-2.0.x (5 params): no cayenne_metastore_wal_truncate_threshold_mb (the background-truncate mechanism landed after v2.0), and cayenne_metastore_wal_autocheckpoint_pages defaults to 10000 in v2.0, not 0. Both differences verified against the v2.0.1 source, so the 2.0.x snapshot documents the 2.0 values, not the trunk values.

Not documented for v1.x — the cayenne_metastore_* params do not exist there (absent from v1.11.0 builder.rs).

Source (per version)

• spiceai/spiceai @ trunk/v2.1.0:
  • crates/runtime/src/builder.rs — reads all 6 keys from runtime.params
  • crates/cayenne/src/metastore/sqlite.rs impl Default for SqliteMetastoreConfig — cache_size_mb: 256, mmap_size_bytes: 1 GiB, busy_timeout_ms: 30_000, wal_autocheckpoint_pages: 0, wal_truncate_threshold_bytes: 160 MiB, auto_vacuum: None
• spiceai/spiceai @ v2.0.1:
  • crates/runtime/src/builder.rs:273-295 — reads 5 keys (no wal_truncate_threshold_mb)
  • crates/cayenne/src/metastore/sqlite.rs default — wal_autocheckpoint_pages: 10_000, no wal_truncate_threshold_bytes field

Test plan

• cd website && npm run build passes
• Versioned-docs propagation: vNext + 2.1.x (6 params) + 2.0.x (5 params, version-correct defaults); v1.x excluded (params absent)
• Files updated: 3

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