Propagate flagUnion into incremental stream/defer buckets and add tests

[benjaie]

Motivation

• Ensure flagUnion correctly reflects pre-populated store entries when building incremental stream/defer buckets so downstream filtering and dependency checks observe error/inhibited flags.
• Prevent regressions where newly-created incremental buckets lost error flags and incorrectly allowed execution paths that should be inhibited.

Description

• Added helper getFlagUnionFromStore(store: Bucket["store"]) which aggregates _getStateUnion() from each ExecutionValue in a store and returns an ExecutionEntryFlags union.
• Wired the helper into incremental bucket creation so created buckets use flagUnion: getFlagUnionFromStore(store) instead of NO_FLAGS in both stream (processStream/_processQueue) and deferred (processSingleDeferred/specs) code paths.
• Imported the ExecutionEntryFlags type where needed and kept existing behaviour otherwise unchanged.
• Added tests grafast/grafast/__tests__/flagUnionIncremental-test.ts that assert error flags (via flagError/TRAP_ERROR) are visible inside newly-created stream and deferred buckets by using a custom FlagUnionCheckStep that inspects _extra._bucket.flagUnion.

Testing

• Added two automated tests in __tests__/flagUnionIncremental-test.ts covering the streaming case and the deferred case, both asserting that no unexpected errors payloads are emitted and the merged data matches expected results; these tests were added to the test suite.
• No test suite was executed as part of this change (tests were created and committed but not run in this rollout).

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Codex Task