refactor: quick-win cleanup — panic stubs, SaveToFile, dead code, error handling, syncWriter

[retran]

Why

The executor and SDK layers have accumulated several code hygiene issues during rapid feature development: panicking stubs that crash at runtime instead of delegating to existing implementations, silently discarded errors that hide data corruption and debugging signals, a background catalog build that never propagates results back to the main executor, a SQL injection vector in SaveToFile, and dead code that adds maintenance burden. This PR addresses all of these as a zero-feature-change cleanup, reducing the risk surface before the upcoming nanoflow and WASM work.

What

• Panic stubs → delegation: 3 SerializeWidget/ClientAction/WorkflowActivity stubs now delegate to existing mpr.Serialize* free functions instead of panicking
• SaveToFile hardening: Escape single quotes in VACUUM INTO path; remove unused destDB open
• Dead code removal: Delete types.Hash + mpr.Hash (zero callers, weak positional checksum)
• Background catalog propagation: Add SyncCatalog callback with sync.RWMutex protection so REFRESH CATALOG BACKGROUND results are visible to subsequent commands; close old catalog on replacement
• Synchronized output: syncWriter (mutex-wrapped io.Writer) prevents interleaved output between background goroutine and main dispatch loop
• Error handling: 7 previously-discarded errors now logged or propagated — updateTransactionID returns error, widgetStmt.Exec propagates, 4 sites get log-and-continue warnings
• refactor: remove ExecContext.executor back-pointer mendixlabs/mxcli#274 review follow-ups: Unexport EnsureSqlMgr, document syncBack cancellation behavior

Verification

• make build && make test && make lint-go passes
• 15 files changed, 106 insertions, 75 deletions
• No behavior changes for correct inputs; new diagnostics for edge cases

[Copilot]

Pull request overview

Cleanup-focused refactor that hardens error handling, removes dead code, replaces panic stubs with real implementations, and addresses background catalog update propagation/concurrency in the executor/catalog paths.

Changes:

• Propagate/record previously-discarded errors (including _Transaction update in MPR v2 writes and widget insert failures) and add warning logs where appropriate.
• Fix background catalog propagation by adding SyncCatalog callback + catalogMu protection, and add a synchronized writer wrapper for background output.
• Remove unused hashing helpers/tests and harden Catalog.SaveToFile by escaping single quotes for VACUUM INTO.

Reviewed changes

Copilot reviewed 15 out of 15 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
sdk/mpr/writer_units.go	updateTransactionID() now returns error and is propagated at call sites.
sdk/mpr/utils.go	Removes unused Hash wrapper.
mdl/types/id.go	Removes unused types.Hash implementation.
mdl/types/id_test.go	Removes tests for deleted Hash.
mdl/executor/executor_dispatch.go	Adds catalogMu usage and SyncCatalog callback wiring; documents cancellation behavior.
mdl/executor/executor.go	Adds catalogMu field to protect background catalog updates.
mdl/executor/exec_context.go	Adds SyncCatalog hook; unexports ensureSqlMgr.
mdl/executor/cmd_sql.go	Updates to call ctx.ensureSqlMgr().
mdl/executor/cmd_pages_builder_v3.go	Logs warnings instead of discarding lookup errors.
mdl/executor/cmd_page_wireframe.go	Logs warning if describePage fails instead of silently discarding error.
mdl/executor/cmd_microflows_builder_calls.go	Logs warning if Java action lookup fails instead of discarding error.
mdl/executor/cmd_catalog.go	Adds syncWriter, background catalog propagation via SyncCatalog, and warns on writeResult failure.
mdl/catalog/catalog.go	Removes unused destination DB open; escapes quotes in path for VACUUM INTO.
mdl/catalog/builder_pages.go	Propagates widget insert errors instead of discarding; wraps with context.
mdl/backend/mpr/backend.go	Replaces panicking serialization stubs with delegation to existing mpr.Serialize* functions.

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Pull request overview

Copilot reviewed 15 out of 15 changed files in this pull request and generated 7 comments.

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Copilot reviewed 15 out of 15 changed files in this pull request and generated 4 comments.

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Copilot reviewed 15 out of 15 changed files in this pull request and generated 3 comments.

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Copilot reviewed 15 out of 15 changed files in this pull request and generated 2 comments.

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Copilot reviewed 90 out of 103 changed files in this pull request and generated 1 comment.

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[retran]

Superseded by upstream PR mendixlabs#287