fix(runtime): report safe startup locator publication failures

[huangruiteng]

When the managed Effect runtime cannot publish its startup locator, an unhandled async listen-callback rejection makes the launcher report only runtime_exited_before_ready. For a locator directory conflict or a live mutation lock, this loses the actionable filesystem/lock reason.

Catch that publication failure and reuse effectRuntimeErrorPayload's existing codes in the already-supported typed startup envelope. Emit a fixed bounded message without raw Node errors, paths, tokens or stack traces. Preserve the original locator/live lock, startup ownership and current retry rules. No new vocabulary, fallback runner, readiness claim or authority repair. The guide and S2/S12 checkpoint document the boundary.

Validation on macOS / Node 24.21.0 / Python 3.12.15:

• Both real fault fixtures fail on main 99839aeb8fed5fae38a5d319391cd050672a6508 with the generic diagnostic, and pass with exact io_is_directory (Unix directory conflict) / mutation_lock_timeout codes. Windows directory replacement retains the shared io_permission_denied classification. They verify a single failed startup, safe stderr, start-lock cleanup and preservation of the original occupant.
• Runtime integration/restart/compile-cache/request-scope suites: 94 passed.
• Native error/IO/fairness tests: 9 passed. Control-plane typecheck and Chat bundle build passed.
• Semantic advisory: no new supported vocabulary carriers; full drift suite: 116 passed.
• Risk-selected premerge: 4 direct + 14 selected passed. Final committed diff check passed.
• Isolated installed-wheel runtime journeys: 4 passed, including both publication failures, unexpected-exit recovery and stale-metadata replacement; all loaded LoopX modules come from the wheel. Metadata version remains 1.2.4; source revision is recorded separately.

This is a reproduced diagnostic repair, not attribution or waiver of an unrelated Linux unexpected-exit CI failure. Imports, bind errors and other exits without a typed envelope can still report runtime_exited_before_ready. Linux/Windows CI and maintainer review remain required. This PR is independent on main; deployment with other unmerged changes requires separate composed-candidate qualification. No production service or formal installation was changed.

Current validation head: a9994b1643aa1df19982563c82ae8be806eaef11. The first Windows run exposed a Unix-only test expectation (1 failed, 172 passed, 6 skipped). Correct the directory fixture to require Windows’ existing permission diagnostic; retain the exact envelope, privacy, readiness and occupant-preservation assertions. No runtime behavior changed in that correction. The 94 runtime tests, premerge 4 direct + 14 selected checks, and a freshly built installed-wheel 4-journey canary pass again on macOS. Earlier native/semantic/typecheck/build evidence covers unchanged runtime code. Exact-head run 37173578474 is now complete: Windows succeeded; Linux JUnit records 15,721 passed, 72 failed and 83 skipped. Both new locator-publication fixtures and the original retry-safe unexpected-exit recovery passed in this run. The prior Linux unexpected-exit failure remains unattributed; this passing sample is not causal proof. Backpressured preview-input cleanup still failed. Overall CI remains failed, and no failure is waived.

Composition checkpoint: local composition of this exact head with public #5546, #5543, #5548 and #5550 on main 99839aeb8fed5fae38a5d319391cd050672a6508 passes 682 targeted Python tests (one upstream settings forward-reference warning), 45 native Core tests, Core/dashboard typechecks, Chat build, Ruff, the repository-configured 19-file Mypy set, and premerge 5 direct + 19 selected checks. A freshly built isolated wheel passes 12 conversation/Agent/status journeys and 4 runtime fault/recovery journeys; native macOS service install/restart, frozen package identity, rejection of unqualified replacements and cleanup also pass with synthetic configuration. An additional expanded Mypy scan following imports fails on 4,436 errors across 533 files; it is not declared passed. These bounded checks do not qualify overall CI or a release.