fix: preserve structured Backend resolver errors - #6

Scope

References Runtime issue #4. This PR is the Runtime-side error-contract follow-up to the resolver-backed extension lifecycle: structured errors raised by the trusted Backend resolver must remain actionable when they cross Lua execution boundaries.

This change is part of the wider CDIN integration and release epic #111, but it does not by itself complete live product-level extension activation.

Changes

• Preserve recognized cdin.runtime error records (domain, stable code and details) when a nested require() module fails.
• Preserve the same records when the entry module executes or its init hook fails.
• Preserve resolver errors raised while Runtime resolves the entry module itself.
• Keep ordinary Lua errors normalized to BAD_EXTENSION.
• Keep sandbox capability denials as PERMISSION_DENIED, rather than flattening them to BAD_EXTENSION.
• Add integration coverage for nested resolver, entry-resolution, entry-execution and init failures; assert failed loads register no extension and partial init contributions are rolled back.
• Define and test deterministic lifecycle behavior: duplicate active loads are idempotent, failed unload hooks still clean owner-scoped contributions, unknown unloads return false, and manager shutdown removes contributions without invoking unload hooks.
• Document resolver error propagation and duplicate/unload/shutdown semantics.

Ownership and integration boundaries

• Runtime owns Lua execution, extension lifecycle policy, owner-scoped rollback and structured Runtime errors.
• Backend issue #14 owns the confined filesystem resolver. Backend PR #16 adds trusted owner-thread C lifecycle entry points and translates Runtime error codes to Backend statuses. The current Backend candidate a906e3265daa6c1aa304a17a1b0dd07c07abfcf8 includes the Lua integration-target build fix and case-insensitive filesystem test correction; its Live Runtime and native/sanitizer CI passed.
• Extension administration remains a trusted local embedding API. This PR does not add extension.load / extension.unload Frontend Protocol commands and does not grant remote Drivers permission to load local code.
• CDIN's Composition Root PR #110 pins this Runtime candidate at 07d43f9b1aba96baf707635c5e821f6da217bb4f and Backend at a906e3265daa6c1aa304a17a1b0dd07c07abfcf8. The combined CDIN native/sanitizer run 42199285019 is pending; product-level capability grant/deny and failure-isolation evidence remains part of issue #111.
• Frontend issue #15 (GUI-SDL adapter) and PR #16 (Windows ConPTY TUI handles) remain Frontend-owned work and require no Runtime API change.

Verification

Runtime CI run 42198326330 passed:

• Linux / Lua 5.5
• macOS / Lua 5.4
• Windows / Lua 5.4
• Documentation and workflow validation
• Pipeline quality task

This PR is intentionally separate from the merged resolver-backed lifecycle work in Runtime PR #5. It references, but does not close, the broader Runtime, Backend or CDIN integration issues.