Add qps-ploc pseudolocalization support

[andrew-polk]

Problem

Applications built on L10NSharp have no good way for testers to verify internationalization. Hard-coded (non-localizable) strings, clipped layouts, broken format placeholders, and runtime string concatenation are invisible in an English UI and only surface after real translations arrive.

Fix

This PR adds the industry-standard qps-ploc pseudo-locale. Any lookup for language qps-ploc returns the live English text pseudolocalized — every vowel doubled with an accent on the first of the pair for ~30-40% in-word expansion, all wrapped in brackets (e.g. Title Missing → [Tîitlée Mîissîing]) — with format placeholders ({0}, {0:n0}, named {app_title}-style, and %0-style) and HTML/XML tags passed through untouched. Plain English on screen then means a hard-coded string; a missing ] means truncation; brackets mid-sentence mean concatenation.

• Lookups work by simply setting the language code (SetUILanguage("qps-ploc") or a per-call language list); all the lookup funnels (static, preferred-languages, dynamic, and the WinForms paths — both runtime lookups and designer-created controls, whose localizers read the string cache directly) are hooked, and the transform is applied exactly once. The code-supplied English wins over the cache, mirroring the existing en rule.
• The pseudo-locale is derived from English at lookup time: no translation files exist, are loaded, or are written for it, it never maps to/from a real language, and it reports itself exactly as complete as English (availability and counts).
• LocalizationManager.OfferPseudoLocalization (default false) gates only whether it is advertised in GetAvailableLocalizedLanguages()/GetUILanguages(), so end users never see the test locale unless the app opts in (e.g. alpha channels). Display name is hard-coded as "Pseudo-English (qps-ploc)".
• The transform lives in an isolated, internal src/L10NSharp/Pseudo/ silo and adds no package dependency. The vowel-doubling style follows Mozilla’s pseudolocalization approach, chosen for readability after trying several styles live in Bloom; the MIT-licensed PseudoLocalizer project was the starting point, and its adapted placeholder/markup-skipping logic remains (attribution in the silo’s README). LocalizationManager.PseudoLocalize(string) exposes the transform for strings that bypass L10NSharp.

Additive only — no behavior change for consumers that never request qps-ploc. 31 new tests pin the transform behaviors and the lookup/gating semantics.

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Devin review

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This change is 

[github-actions]

Test Results

    7 files  ±    0  144 suites  +20   27s ⏱️ +3s
218 tests +  31  213 ✔️ +  31    5 💤 ±0  0 ❌ ±0 
838 runs  +124  823 ✔️ +124  15 💤 ±0  0 ❌ ±0 

Results for commit 23cd79b. ± Comparison against base commit 36f0151.

♻️ This comment has been updated with latest results.

[andrew-polk]

[Claude Fable 5] Consulted Devin on 2026-08-21 up to commit 139d6fe. Across three review rounds it raised two bugs — an English-over-pseudo preference edge (fixed in c436838 before it reached GitHub) and a null preferred-language-id crash (fixed in 139d6fe, thread, resolved) — plus 6 informational notes assessed as not actionable. Final re-review is clean: 0 bugs, 0 investigate flags. CI green on all checks.

[andrew-polk]

[Claude Fable 5] Consulted Devin on 2026-08-24 up to commit dd7f3a0 (the switch to the doubled-vowel transform). Re-review clean: 0 bugs, 0 investigate flags, 7 informational notes assessed as not actionable. CI green on all checks.

[andrew-polk]

[Claude Fable 5] Consulted Devin on 2026-08-24 up to commit b5b346d. Re-review clean: 0 bugs, 0 investigate flags, 9 informational notes assessed as not actionable. CI green on all checks.

[andrew-polk]

[Claude Fable 5] Consulted Devin on 2026-08-24 up to commit 303b495 (the WinForms designer-path fix and the master merge). Re-review clean: 0 bugs, 0 investigate flags, 9 informational notes assessed as not actionable. CI green on all checks.