desktop(cleanup): post-v1 legacy shell retirement boundary

[Whiteks1]

Status

Post-v1 cleanup boundary. Do not execute this issue as broad legacy deletion yet.

After #442, Desktop v1 is defined as a functional operator workstation with explicit transitional boundaries:

• legacy remains the default release runtime where still required for complete operator flow
• React is a validated selectable runtime and the canonical future direction
• React is not yet the default release path

Therefore #412 is no longer a direct "delete legacy now" task. It is the final cleanup target after the remaining release/runtime boundaries are resolved or split into narrower issues.

Objective

Retire the legacy shell renderer only after the Desktop no longer needs it for required release flow, or after every remaining dependency has been explicitly preserved as an external API boundary.

This issue should close the old legacy-retirement intent, but only once the preconditions below are satisfied.

Preconditions before execution

• React default/runtime decision is explicit and validated, or legacy is no longer required for release flow.
• Launch submission has either a native contract or an intentionally documented transitional API boundary.
• Job / Launch Review remains React-owned and does not depend on legacy renderer ownership.
• research_ui dependencies are classified as external API, transitional continuity, or safe to delete.
• Smoke coverage no longer relies on legacy renderer assumptions for the release definition.
• Browser-backed links such as Open research_ui, Browser ops, and Browser view are not primary operator actions.

Recommended split before cleanup

Before implementing this issue, split or verify narrower slices such as:

• React/default runtime release decision
• native Launch submission contract or explicit API boundary
• smoke validation aligned with the v1/runtime decision
• removal of only proven-unused legacy/browser-backed paths

Done when

• Legacy renderer code is removed only where no required release path depends on it.
• Any remaining research_ui usage is explicitly preserved as an API/reachability dependency, not product ownership.
• React/default runtime state is documented and validated.
• npm run -s typecheck is green.
• relevant desktop smoke checks are green.
• No broad product, Launch, Stepbit, Quant Pulse, or ML work is mixed into this cleanup.

Out of scope

• No broad deletion before the preconditions are satisfied.
• No React default switch unless handled by a dedicated issue.
• No Launch expansion.
• No research_ui server removal while APIs are still required.
• No feat(desktop): add minimal hypothesis builder #266 / hypothesis-builder work.
• No N.1 / ML implementation work.

[Whiteks1]

#412 should remain the final cleanup target, not the next executable deletion slice.

Current blocker issues created before executing legacy retirement:

• desktop(react): make React shell own migrated workstation surfaces #430 — make React shell own migrated workstation surfaces without app-legacy.js dependency.
• desktop(test): make smoke validation React-runtime ready #431 — make smoke validation React-runtime ready instead of hard-requiring rendererMode === "legacy".
• desktop(product): define research_ui cutoff before legacy retirement #432 — define the final research_ui continuity cutoff before legacy retirement.

Until those are resolved, removing legacy.html, app-legacy.js, tab-renderers.js, useLegacyBridge.js, or research_ui would likely break the current desktop runtime, smoke path, Launch/Job continuity, or v0.1 release surface.

Recommended interpretation: keep #412 as the final cleanup issue after React runtime ownership and validation are real.