Add Plugin-Shared library + AmericanToBritish & BritishToAmerican plugins

[niksedk]

Summary

`se5/Plugin-Shared/` (new library)

Small Avalonia 11 / .NET 8 class library that SE5 plugins reference instead of copy-pasting boilerplate.

• `PluginContract.cs` — request / response / subtitle DTOs.
• `PluginApp` — `Application` base that reads `request.theme` and sets `RequestedThemeVariant` so plugins automatically match SE's Dark / Light theme.
• `PluginBootstrap.Run(args)` — parses the request JSON from the first command-line argument, starts Avalonia with classic desktop lifetime, writes the response on exit, returns 0. Each plugin's `Program.cs` becomes a single line.
• `SubRipParser` — lifted out of TypewriterEffect.
• `WindowExtensions.BringToForeground()` — the `Topmost`-toggle pattern used to force the plugin window in front of SE on macOS.
• `EnglishVariantConverter` (new) — the US↔BR regex word-list engine, with a `Direction` enum.
• `WordList.xml` (embedded resource) — the ~1000-pair word list, originally in AmericanToBritish.

Word-list review + growth

• Bug fix: `anticlockwise` / `counterclockwise` was reversed (anticlockwise is the British form). Swapped.
• +225 entries (1013 → 1238 pairs), mostly the ~50 most common `-ize`/`-ise` verbs missing from the original list (memorize, scrutinize, finalize, modernize, summarize, specialize, sympathize, moisturize, prioritize, normalize, legalize, visualize, terrorize, traumatize, hospitalize, humanize, stabilize, mobilize, neutralize, synthesize, utilize, etc.) with all four conjugations each. Also added math/maths, fetus/foetus (+ fetal, fetuses), and a small set of unambiguous vocab (truck/lorry, elevator/lift, apartment/flat, sidewalk/pavement, flashlight/torch, gasoline/petrol, eggplant/aubergine, zucchini/courgette, diaper/nappy).

`se5/AmericanToBritish/` (new plugin)

Faithful port of the SE4 American to British converter (regex word list with lowercase / UPPERCASE / Titlecase variants + the `color`-in-``-tag revert). UI is a clean Avalonia preview window:

• Title + scope summary ("Convert American to British English spellings in the X selected line(s)…").
• A list of every proposed change — each row has a checkbox, line number, the original text (muted), and the converted text (SemiBold).
• Select all / Select none buttons, a live "X of Y change(s) selected" summary, Cancel, and an accent-styled Apply (disabled when nothing is checked).
• Empty list ⇒ a centered "No American-to-British conversions found in the selected lines." instead of an empty box.
• Theme variant comes straight from `request.theme`.

`se5/BritishToAmerican/` (new plugin)

Mirror of AmericanToBritish using `Direction = BrToUs`. Same UI shell, same shared converter, same word list.

Build + index

• `.github/workflows/american-to-british.yml` and `british-to-american.yml` mirror `typewriter.yml`: matrix self-contained publish for the six supported RIDs, plugin.json rewritten per-platform with the `executables` block, six zips uploaded as artifacts, and a GitHub release on manual dispatch with a tag. Both `paths` filters watch `se5/Plugin-Shared/**` too.
• `se5-plugins.json` extended with AmericanToBritish + BritishToAmerican entries pointing at the planned `se5-american-to-british-v1.0` / `se5-british-to-american-v1.0` release assets via per-platform `downloads` maps.

Out of scope (next PR)

• Retrofitting TypewriterEffect onto Plugin-Shared.
• WordCensor plugin.
• OneDrive Load / Save — separate effort.

Test plan

• CI matrix builds pass on all six RIDs for both plugins.
• After merge, dispatch each workflow with the appropriate tag to publish the releases.
• In SE: install both via Get plugins online... → run AmericanToBritish on a subtitle with US spellings → preview shows changes → Apply → status shows count.
• Same for BritishToAmerican on a subtitle with UK spellings.
• Confirm both windows match SE's theme.

🤖 Generated with Claude Code