Take libpalaso 18, which recovers an abandoned SLDR mutex

[johnml1135]

A process killed while holding libpalaso's machine-wide SldrCache mutex made every later writing-system load on the machine fail with AbandonedMutexException — Motif's, and FieldWorks' too — until something took and released it. One run of our suite lost 445 tests to it. libpalaso fixed it in sillsdev/libpalaso#1544 (LT-21834), shipped in SIL.Core/SIL.WritingSystems 18.0.0-beta0042; this takes that release.

What changes

• SilVersions.props pins libpalaso 18.0.0-beta0042. SIL.LCModel (even 11.0.0-beta0182) still asks for 17.x, so the SIL.Core/SIL.WritingSystems pins in five projects are now unconditional instead of applying only under the local-libpalaso override.
• AbandonedSldrMutexTests pins the new behaviour: it abandons a privately named GlobalMutex and takes it again. It fails on 17.0.0-beta0097 with AbandonedMutexException and passes on 18.0.0-beta0042. It uses a private name on purpose — abandoning the real SldrCache mutex hangs any other process still on 17.
• AGENTS.md says why the pin sits above what LCModel asks for.

Evidence and risk

• ./test.ps1 -Configuration Release: 2,021 passed, 0 failed, 40 skipped.
• Risk: 18 is a major version above LCModel's dependency. libpalaso's major bump came from SIL.Windows.Forms (ImageCropper, +semver:major Fix unusable ImageCropper crop; RawFormat now MemoryBmp libpalaso#1530), which Motif does not reference; the full suite is the compatibility evidence.
• libpalaso 18 also writes the SLDR cache and the writing-system store atomically, so an interrupted write no longer leaves a truncated file.

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