refactor(desktop): share the repeated codec, engine and frontend helpers

[bobzhang]

This half needs a careful read. Every hunk here adds or removes something
SHARED, so correctness depends on code outside the hunk you are looking at. The
purely in-function rewrites this was mixed with have moved to a separate pull
request, so what is left is all shared-surface change.

What to check, hardest first:

1. An extracted helper must be equivalent at every call site it replaced.
   The trap is a call site that passed a different argument, or read state at a
   different moment, than its neighbours.
2. A merged or-pattern arm must have had identical bodies. Where the arms
   differed and the difference moved inside the arm, the fold buys less than it
   appears to.
3. A reshaped loop must keep its iteration order and its early exits.

Scope: desktop/. The largest pieces are the protocol codec readers and writers each boundary had copied, the engine's shared refusal and run-actor fixtures, and the frontend's repeated update transitions.

Review comment addressed

[for key, _ in ... if !desired_keys.contains => key] — applied. The stale
editor keys are filtered in the comprehension, so the intermediate array of
every editor key is gone. The predicate is named rather than inline because
moon fmt wraps the longer form, and a wrapped comprehension whose body is a
bare identifier becomes { key }, which the compiler reads as a struct literal.

Verified on this branch alone

moon fmt --check, moon check --target native --deny-warn, moon check --target js --deny-warn, and both full test suites, with no other part of the
refactor applied. No .mbti changed.

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The refactor is split so each pull request asks one kind of question. Every file
appears in exactly one of them, each was verified on its own branch with no
other part applied, so they merge in any order.

	easy to review	needs a careful read
root	#1330	this one (#1325)
desktop/	#1331	#1326
editor/	#1332	#1327
tests	#1333	#1328

🤖 Generated with Claude Code

https://claude.ai/code/session_01JiBNGzCsZpN6e5Y4p9pXdn

[Copilot]

🔵 Needs a closer look

The broad behavioral refactor spans many critical subsystems, and the final changed file’s diff is truncated, preventing a complete review.

Pull request overview

Refactors repeated codec, engine, host, worktree, and frontend logic into shared helpers while preserving existing behavior.

Changes:

• Consolidates JSON encoding/decoding and repeated lookup/error-handling patterns.
• Extracts common engine/worktree test fixtures and rollback/refusal helpers.
• Simplifies frontend state transitions, editor rendering, and UI construction.

File summaries

File	Description
desktop/internal/worktree/worktree.mbt	Simplifies lookup and extracts worktree test setup.
desktop/internal/protocol/text_search_codec.mbt	Shares numeric and error-pair decoding.
desktop/internal/protocol/semantic_search_codec.mbt	Reuses shared codec validation helpers.
desktop/internal/protocol/json_object.mbt	Adds generic optional encoding and typed decoding.
desktop/internal/protocol/git_protocol_codec.mbt	Applies shared JSON helpers.
desktop/internal/protocol/desktop_replies.mbt	Consolidates optional field codecs.
desktop/internal/protocol/desktop_messages.mbt	Consolidates message encoders and decoders.
desktop/internal/protocol/codex_commands.mbt	Applies generic codec helpers.
desktop/internal/protocol/agent_payload_codec.mbt	Simplifies optional and array codecs.
desktop/internal/moonbit/toolchain_paths.mbt	Deduplicates fixture file creation.
desktop/internal/host/update_check.mbt	Consolidates update availability calculation.
desktop/internal/host/text_search.mbt	Extracts surrogate checking and first-line searches.
desktop/internal/host/terminal_ops.mbt	Removes redundant private payload types.
desktop/internal/host/host.mbt	Emits terminal payloads directly.
desktop/internal/host/git_ops.mbt	Shares Git output/error helpers and simplifies classification.
desktop/internal/engine/worktree_seam_wbtest.mbt	Reuses engine fixture and refusal helpers.
desktop/internal/engine/ops.mbt	Adds shared run-test fixture setup.
desktop/internal/engine/follower.mbt	Reuses refusal capture helper.
desktop/internal/engine/engine.mbt	Simplifies map initialization, collection, and event emission.
desktop/internal/engine/config.mbt	Reuses refusal capture helper.
desktop/internal/engine/archive.mbt	Shares rollback and test refusal logic.
desktop/internal/engine/archive_lookup_wbtest.mbt	Reuses refusal capture helper.
desktop/internal/codex/actor.mbt	Shares actor request submission flow.
desktop/internal/auth/urls.mbt	Extracts suffix removal.
desktop/internal/auth/signin.mbt	Reuses fallback and URL helpers.
desktop/internal/auth/pkce.mbt	Simplifies Base64URL conversion and URL handling.
desktop/frontend/transcript/sessions.mbt	Consolidates terminal projection control flow.
desktop/frontend/transcript/component/view_tests.mbt	Reuses base test input fixtures.
desktop/frontend/transcript/component/tool_tabs.mbt	Simplifies argument and output tab construction.
desktop/frontend/storage.mbt	Shares guarded local-storage mutation.
desktop/frontend/skills/state.mbt	Extracts preview cache/fetch transition.
desktop/frontend/project_picker/requests.mbt	Shares browse failure construction.
desktop/frontend/model.mbt	Simplifies searches and transcript reload transitions.
desktop/frontend/goal.mbt	Extracts repeated runtime-notice test setup.
desktop/frontend/fileeditor/view.mbt	Shares focus, surface, and graph rendering helpers.
desktop/frontend/fileeditor/update.mbt	Consolidates review reconciliation and test fixtures.
desktop/frontend/fileeditor/semantic_review_view.mbt	Shares semantic editor disposal.
desktop/frontend/fileeditor/editor_panel.mbt	Consolidates viewer-model cleanup.
desktop/frontend/composer/component.mbt	Merges repeated reconciliation and completion paths.
desktop/frontend/composer/codex_request.mbt	Shares request decision button construction.
desktop/frontend/codex/bridge.mbt	Shares textual mention fallback and request lookup.
desktop/frontend/bridge.mbt	Consolidates recovered-run and failure transitions.
desktop/frontend/goal.mbt	Diff excerpt is incomplete, preventing full review.

Review details

• Files reviewed: 43/43 changed files
• Comments generated: 0
• Review effort level: Balanced

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[bobzhang]

[ for key,_ in ... if !desired_keys.containts => key]

[bobzhang]

Applied. The stale keys are filtered in the comprehension, so the intermediate
array of every editor key is gone.

One wrinkle worth recording: written inline the line is long enough that
moon fmt wraps it, and a wrapped comprehension whose body is a bare
identifier gets reformatted to { key }, which the compiler then reads as a
struct literal (E4033, "There is no record definition with the fields: key").
Naming the predicate keeps it on one line and avoids that.