tests: fix String→TabID mismatch in WorkspaceLayoutExecutorAcceptanceTests

[BenevolentFutures]

Summary

Unblocks `mailbox-unit` (`mailbox-parity.yml`) on every PR/push that triggers it. The compile error was already there but masked by SentryHelper.swift's MetricKit issue; #114 cleared that and this surfaced as the next layer.

What's broken on `origin/main`

```
c11Tests/WorkspaceLayoutExecutorAcceptanceTests.swift:480:68: error:
cannot convert value of type 'String' to expected argument type 'TabID'
c11Tests/WorkspaceLayoutExecutorAcceptanceTests.swift:500:13: error:
conflicting arguments to generic parameter 'T' ('String?' vs. 'TabID')
```

`ExternalTab.id` is a stringified UUID (bonsplit's external view) but `Workspace.panelIdFromSurfaceId(:)` and `surfaceIdFromPanelId(:)` are typed on the opaque `TabID` wrapper. The `comparePane` helper:

• passes `tab.id` (String) where `TabID` is expected, and
• compares `livePane.selectedTabId` (String?) against `expectedTabId` (TabID) inside `XCTAssertEqual`.

Fix

Apply the same conversion `WorkspacePlanCapture.panelID(forTabIDString:)` uses at the v2 socket boundary: parse the UUID string and wrap with `TabID(uuid:)`. For the equality assertion, project the TabID wrapper to its canonical UUID-string form so both sides of `XCTAssertEqual` are in the same value space.

Why this didn't catch locally before

Two reasons: (1) `xcodebuild build` on the `c11-unit` scheme only builds the host app target — `build-for-testing` (or `test`) is what compiles the test target; (2) Local Xcode 26 / Swift 6.2 happens to accept the old code through some inference path that Xcode 16.4 / Swift 6.1 in CI does not. The fix compiles cleanly on both.

Why no one else has hit this

`mailbox-parity.yml` only triggers when `Sources/Mailbox/**`, `CLI/c11.swift`, etc. change. Most main pushes don't touch those paths so the workflow doesn't run, so this regression has been quietly sitting on main since the TabID wrapper was introduced.

Test plan

• Local build clean (the file now compiles without error on Swift 6.2)
• CI `mailbox-unit` job goes green for the first time since the breakage

🤖 Generated with Claude Code