Add live as-you-type diagnostics for the cargo backend

[crisidev]

Forgot to push this yesterday night :)

Opt-in via init_options.cargo.updateOnInsert = true. When enabled, the server hardlinks a shadow copy of the workspace under target/bacon-ls-live/, mirrors dirty buffers from textDocument/didChange into the shadow, and runs cargo there with --remap-path-prefix so diagnostics point at the user's source. Coalesces keystrokes via a configurable debounce (cargo.updateOnInsertDebounceMillis, default 500ms).

The capability has to come through init_options rather than workspace/configuration because the static textDocument/didChange sync mode must be advertised in the initialize response, and clients (Neovim in particular) don't reliably retrofit attached buffers when the server tries to register that capability dynamically post-initialized.

Also redirect the global tracing subscriber set up by test_can_configure_tracing to a tempfile so cargo test no longer clobbers the developer's bacon-ls.log.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 76.79671% with 113 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.21%. Comparing base (144fe76) to head (aaed552).

Files with missing lines	Patch %	Lines
src/lib.rs	65.94%	57 Missing and 6 partials ⚠️
src/shadow.rs	88.70%	10 Missing and 18 partials ⚠️
src/lsp.rs	59.25%	22 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #123      +/-   ##
==========================================
+ Coverage   79.03%   79.21%   +0.18%     
==========================================
  Files           5        6       +1     
  Lines        2766     3209     +443     
  Branches     2766     3209     +443     
==========================================
+ Hits         2186     2542     +356     
- Misses        489      554      +65     
- Partials       91      113      +22     

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