Replace silent CREATE PROTO BUNDLE ignore with --ignore-proto-bundles flag

[daichirata]

Summary

CREATE PROTO BUNDLE was silently dropped in NewDatabase, while the analogous unsupported statement CREATE MODEL required users to opt in via the --ignore-models flag. This PR unifies the two patterns under the explicit-opt-in approach by introducing --ignore-proto-bundles.

Problem with the silent path

// Before (internal/hammer/diff.go)
case *ast.CreateProtoBundle:
    // CREATE PROTO BUNDLE is not supported, ignore it.

This has two issues:

• Users can't tell that hammer just stripped a statement from their schema, so a CREATE PROTO BUNDLE typo or paste mistake never produces an error.
• If hammer later grows real CREATE PROTO BUNDLE support, schemas that relied on the silent drop will suddenly start emitting diffs the user never asked for.

Changes

• DDLOption gains an IgnoreProtoBundles bool field.
• ParseDDL filters *ast.CreateProtoBundle when the flag is set, matching the existing *ast.CreateModel handling.
• The silent case *ast.CreateProtoBundle: in NewDatabase is removed, so the statement now falls through to the default unexpected ddl statement error path.
• --ignore-proto-bundles is added to apply / create / diff / export.
• The three ignore create proto bundle in {from,to,both} tests set ignoreProtoBundles: true and continue to verify the flag's behaviour.
• README's Flags section lists the new flag.