fix(fsharp): hover honors the live editor buffer (didChange overlay)

[MelbourneDeveloper]

TLDR

F# hover (and every per-file F# analysis) now resolves against the live editor buffer instead of stale on-disk text, fixing broken/empty F# hover that occurred the moment you started typing.

Details

Root cause. The F# sidecar was excluded from document sync. notify_did_change in src/main.rs was hardcoded to the C# sidecar, the F# sidecar registered no textDocument/didChange handler, and FSharpWorkspace.getHover read source via File.ReadAllText. Once the editor buffer diverged from disk, the editor's line/char pointed into edited text while FCS parsed the old file — so hover returned the wrong symbol or null. C# was unaffected because Roslyn's workspace is updated in place on didChange; this restores F# to parity (CLAUDE.md aim #2).

Modified behaviour

• src/main.rs: new sidecar_for_uri helper. didOpen/didChange now route the text sync to the document's own sidecar by language (F#→F#, C#→C#) — previously every edit went to the C# sidecar. pick_sidecar and trigger_diagnostics now reuse sidecar_for_uri, removing the duplicated language→sidecar match.
• sidecars/SharpLsp.Sidecar.FSharp/FSharpWorkspace.fs: FSharpWorkspaceState gains a thread-safe Overlays map; new applyDidChange + readSource. getHover and checkFileWithParse read source via readSource (so hover, definition, completion, signature help, … all see live edits), falling back to disk only when no buffer is open.
• sidecars/SharpLsp.Sidecar.FSharp/FSharpSidecar.fs: registers textDocument/didChange → applyDidChange.

New files / types / deps: none. New wire type DidChangeRequest in FSharpWire.fs (positional MessagePack, mirrors the Rust SidecarDidChangeReq and C# DidChangeRequest). No new dependencies. No deletions.

Spec: docs/specs/HOVER-SPEC.md §5.4 documents the live-buffer requirement, tagged [FS-DIDCHANGE-OVERLAY] (cross-referenced in the code and tests).

How Do The Automated Tests Prove It Works?

• New full-stack regression e2e_modules::fsharp::test_full_stack_fsharp_hover_reflects_live_edit: opens the F# fixture, sends a didChange adding subtractEdited (a binding that exists only in the buffer, never on disk), then hovers on it. Verified test-first: before the fix it fails — host log shows Sidecar response … method=textDocument/hover … bytes=1 and Hover: sidecar returned null; after the fix it passes — method=textDocument/didChange … bytes=3 (ok) followed by Hover: sidecar returned content.
• New sidecar-level coverage in FSharpExtraCoverageTests.fs:
  • getHover reads the didChange overlay instead of on-disk source — isolated real workspace, overlays a file with overlayOnlyBinding, asserts getHover resolves it from the overlay (proving applyDidChange + readSource).
  • textDocument/didChange added to the malformed-payload Theory, exercising the handler's with ex -> Failure branch (and that the sidecar still answers ping afterward).
• Full suites green locally: Rust e2e 640 passed, 0 skipped (coverage 94.65%, gate ✓), incl. test_did_change_then_definition_exercises_notify_path and test_definition_cache_invalidated_on_change. F# sidecar 323 passed, C# 394, Common 63 (coverage F# 94.77% / C# 94.16% / Common 96.02%, all gates ✓). C# hover module 8 passed incl. test_hover_edit_hover_cycle (no C# regression). document_sync 13 passed. _lint-rust (fmt + clippy -D warnings), csharpier check, and _lint-dotnet (0 warn / 0 err, warnaserror) all clean; deslop reports no new duplication.