chore(tests): type-check test sources without Xcode

[StuBehan]

Problem

swift test needs the XCTest module, which only ships with full Xcode. On a
Command Line Tools-only machine the whole test target fails to load — so the test
sources are the one part of this repo that never gets compiled locally.

That has a concrete cost. On #139, tightening series() to require a source:
parameter broke 12 call sites in UsageHistoryTests.swift. Every production file
had been type-checked; the test file could not be, so swift test failed in CI on
a pure build error. Signature-tightening changes are exactly the ones that break
callers you cannot see.

Change

make typecheck-tests compiles the test sources in-module against the real panel
types, with XCTest swapped for stand-ins in scripts/xctest-shim.swift. Takes a
few seconds and would have caught the above before pushing.

It does not run assertions — make test and CI remain the authority on whether
tests pass. Not wired into CI, which runs the real swift test and subsumes this.

How it works

• Test sources are staged to a temp dir with import XCTest replaced by
  import AppKit / import SwiftUI, and @testable import dropped since
  everything compiles as one module.
• panel/main.swift is excluded so its top-level code doesn't clash; build.sh
  type-checks that file as part of the real build.
• Warnings are suppressed: panel/ carries a stack of pre-existing macOS 14
  deprecation warnings that would bury the errors this exists to surface.
• Diagnostics are captured and the staging paths rewritten back to
  Tests/StackNudgePanelCoreTests/, so errors name the file you actually edit
  rather than a temp dir that no longer exists.

Shim fidelity

Four details produced false errors before being fixed, all worth knowing if the
shim ever needs extending:

• Replace import XCTest, don't delete it — XCTest re-exports Foundation, and
  several test files rely on that for Calendar / TimeZone.
• XCTAssertEqual(_:_:accuracy:) needs its own FloatingPoint overload
  (EventListenerTests uses it).
• XCTestCase needs setUp / tearDown / setUpWithError / tearDownWithError.
• Those hooks must be @MainActor, matching real XCTest — otherwise a @MainActor
  test class's override becomes nonisolated and can't touch its own properties.

If the suite starts using an undeclared XCTAssert variant, the type-check fails
with "cannot find 'XCTAssertSomething' in scope"; the fix is to add the overload.

Testing

• Passes across all 26 current test files.
• Failure path verified by injecting the exact breakage from feat(panel): usage history graph pane #139 (a call missing
  the required source: argument): exits 1, and names
  Tests/StackNudgePanelCoreTests/… rather than the temp path.
• bash -n and shellcheck --severity=warning clean, so the new script satisfies
  the checks CI already runs over tracked shell.
• swift build and ./build.sh both unaffected by the new scripts/ directory —
  the shim is never compiled into the app, since both take their sources from
  panel/ and shared/ only.