Function decode 

pub fn decode<T: Document>(
    bytes: &[u8],
    expected_collection_id: u32,
) -> Result<T>

Decode an on-disk record into a T: Document instance.

Validates the per-document header at the runtime boundary (power-of-ten Rule 5):

1. bytes.len() >= DOC_HEADER_SIZE.
2. header.collection_id == expected_collection_id.
3. bytes.len() == DOC_HEADER_SIZE + header.payload_len.
4. CRC32C of the payload matches header.payload_crc32c.
5. header.type_version <= T::VERSION (no future versions).
6. If header.type_version < T::VERSION, the codec consults T::historical_schemas() for the matching version, walks the payload through that schema via Dynamic::from_postcard_bytes, and dispatches into Migrate::migrate with the structured Dynamic (M10 #83). If no schema is registered for header.type_version, the codec returns Error::SchemaNotRegistered without invoking migrate — silent fallback hides schema-evolution bugs. Otherwise (versions match) the payload is decoded directly via postcard::from_bytes::<T>(payload).

Errors

• Error::Corruption with page_id = 0 on a malformed header or CRC mismatch (the codec does not know the page id; callers that need a specific id should wrap or re-emit).
• Error::CollectionIdMismatch on a collection-id mismatch.
• Error::SchemaVersionFromFuture on a stored record newer than T::VERSION.
• Error::SchemaNotRegistered when the stored type_version is older than T::VERSION and T::historical_schemas() has no entry for it.
• Error::SchemaMigrationNotImplemented when the registered Migrate::migrate body returns the default error.
• Error::SchemaTypeMismatch / Error::SchemaDepthExceeded on a schema / payload disagreement.
• Error::Codec on postcard decode failures.