[WRONG BRANCH] fix: distinguish unmapped prompt layers from base prompt in probe and UI

[luvs01]

Motivation

• The prompt-text probe previously labeled several non-base layers as not-exposed, which caused the GUI to show a base-prompt-specific explanation for unrelated layers.
• This produced misleading operator-facing copy when a probe succeeded but the extractor had no confirmed tag mapping for a layer.

Description

• Add a distinct probe reason unmapped to LayerText and return it for the UNMAPPED_LAYER_IDS instead of reusing not-exposed.
• Keep not-exposed for the actual base prompt which is confirmed to travel outside the printable message list.
• Update the read-only PromptLayerDialog to render a separate codexSet.dialog.unmapped translation when a layer is unmapped.
• Add localized unmapped copy across all supported locales and add two focused regression tests that assert the probe classification and the dialog text selection.

Testing

• Ran bun run typecheck, which completed successfully.
• Ran the focused GUI spec cd gui && bun test tests/codex-set-prompt-layers.test.tsx, which passed including the new regression (16 pass).
• Ran cd gui && bun run lint:i18n, cd gui && bun run lint, and cd gui && bun run build, all of which succeeded (build produced dist/ artifacts).
• Attempted bun test tests/codex-prompt-route.test.ts, which failed to initialize under the environment's Bun 1.2.14 due to node:zlib.zstdDecompressSync not being available, so the route-level suite could not be fully executed here.
• Attempted the full GUI test suite (cd gui && bun test tests), which exercised many tests and included the new changes, but the full run showed unrelated timer-based failures and initialization errors under the installed runtime; the focused tests and build remain green.

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Codex Task

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[github-actions]

✅ Deterministic PR hygiene checks passed.

[github-actions]

⏳ DRAFT

• wrong target branch (main); retarget to dev. UI screenshot required.

What to do

• Retarget this PR to dev — all contributions go to dev.
• Add a screenshot of the UI change to the PR description.

Its title has been prefixed with [WRONG BRANCH].
This pull request is being kept as a draft automatically. Once every issue above is resolved, it will be marked ready for review again.