LSP Phase 3: file lifecycle + push diagnostic state + wait API

src/lsp/client.rs (LspClient wrapping RpcClient)
- notify_open(path): reads file, sends textDocument/didOpen on first
  contact or textDocument/didChange (with bumped version) thereafter.
  Returns the version sent so callers can correlate later push events.
- diagnostics_for(path): merged + deduped push/pull diagnostics for one
  file. dedupe key: range + severity + code + source + message
  (matches opencode). Code is load-bearing — same-line same-message
  diagnostics with distinct clippy codes (needless_clone vs
  redundant_clone) must NOT collapse.
- all_diagnostics(): aggregate across every tracked file; empty entries
  pruned.
- wait_for_push(path, after, timeout): blocks until a fresh push for
  path arrives strictly after the given Instant. Subscribes to a watch
  channel that the publishDiagnostics handler bumps on every push;
  re-checks state under each wake. Stale pushes (already in state at
  call time) do NOT satisfy the wait — only fresh arrivals count.
- Push state populated by an internally-registered
  textDocument/publishDiagnostics handler. URIs decoded back to
  PathBufs via percent_decode so we round-trip cleanly with
  path_to_file_uri_string.

15 tests including regressions:
- notify_open second call sends didChange (not another didOpen) with
  bumped version
- dedupe preserves entries with distinct  fields
- wait_for_push ignores stale arrivals (rejects pushes before )
- merged_diagnostics dedupes identical entries

Code review fixes applied before push:
- Dedupe key was missing  — added with regression test.
- Doc-noted blocking std::fs I/O in notify_open for Phase 4 to revisit.

Phase 1: 29, Phase 2: 31, Phase 3: +15 → 75 LSP tests. Suite: 365 -> 380.

Author: Yogthos