Port completion popup window from upstream getline.cs

[vbfox]

Summary

Ports the last cluster of upstream changes to mcs/tools/csharp/getline.cs (Jan–Mar 2016), which predate this F# port and were never carried over:

• a7a4958 — "Add support for completion popup windows" (the core feature)
• 642b064 — fix the top_item scroll logic in the popup
• f91bb07 — call HideCompletions() after auto-inserting a single match
• aee167f — demo update showing off the new heuristics mode

What changed

• CompletionState now renders an actual popup window below the cursor instead of dumping every match as flat text.
• GetLineSettings.HeuristicsMode (Some "csharp") triggers completion automatically (e.g. on .) without waiting for Tab.
• Up/Down (and Ctrl+P/Ctrl+N) navigate the popup instead of history while it's open; Escape closes it; most other edits close it automatically — Handler now carries a ResetCompletion flag, mirroring upstream's per-key resetCompletion parameter.
• CmdDone (Enter) inserts the highlighted completion instead of submitting the line when the popup is open.
• The two follow-up bugfixes (642b064, f91bb07) are folded directly into the initial implementation rather than ported bug-for-bug.
• TestApp's demo now sets HeuristicsMode and shows a popup automatically when typing ., per aee167f.

Bugs found & fixed along the way

While rewriting the common-prefix matching logic that this feature reuses, found two bugs in the existing F# port (not present upstream):

• A missing loop increment that could hang indefinitely when two or more completions shared a prefix character.
• A missing short-circuit that could index a shorter completion string out of bounds (upstream avoids this via goto mismatch, which the original transcription into F# didn't replicate).

Both are covered by regression tests (see below) — I confirmed each actually reproduces the original bug (hang / wrong result) by temporarily reverting the fix and re-running the suite.

Tests

Upstream getline.cs has no test directory of its own, so there was nothing to port there. Added a BlackFox.FsGetLine.Tests project (Expecto, matching the convention in BlackFox/FoxSharp) covering what's actually testable without a real interactive console — Console.ReadKey/cursor APIs make the main edit loop impractical to unit test:

• CompletionState's popup scrolling math, including a regression test that fails under the pre-642b064 formula
• The common-prefix matching fix
• The C# dot-heuristic (numeric literals vs. identifiers)

Wired into the FAKE build as a RunTests task that CI now depends on (previously a DoNothing placeholder in the old build.fsx).

Test plan

• dotnet build on the full solution
• Full clean-room ./build.sh CI (restore → build → run tests → pack) succeeds locally, all 23 tests pass
• Reverted each of the two found bugs individually and confirmed the corresponding test fails (one as an assertion failure, the other as an actual infinite loop caught by timeout)
• Drove TestApp under a real pty: Tab-triggered popup, arrow-key navigation, Escape-to-close, and the heuristic auto-trigger on . all work end to end with no exceptions
• GitHub Actions CI green on this PR (Linux + Windows)

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