Add symbol extraction, inline CodeLens, restore undo, native diff, and fixes

[KirtiJha]

Symbols (#1):

• Capture now records the functions/classes a change touched via VS Code's
  document symbol provider (executeDocumentSymbolProvider), plus the active
  function/class. This activates symbol history, the clickable symbol tags, and
  symbol-aware search/embeddings — previously the symbols array was always empty.

Duplicate-embedding fix (#2):

• Embeddings were generated on CHANGE_CAPTURED, before the row was inserted, so
  updateEmbeddingId() no-op'd and embedding_id stayed NULL -> the change was
  re-embedded (a duplicate vector) on every restart. Now embeds on a new
  CHANGES_FLUSHED event, after insert.

Undo on restore (#3):

• RestoreResult carries the backup id; a native notification offers one-click
  Undo (wired to the existing undoRestoration).

Inline history CodeLens (#4):

• New provider shows "N changes in history" above a file and "k versions" above
  each function/class; click to open the file/symbol history. Gated by
  ui.showInlineHistory. Fixed showFileHistory (was matching an absolute path
  against relative-path records).

Native diff compare (#5):

• "Compare with current" opens VS Code's diff editor (file now <-> before this
  change); "Open diff" shows the change's unified diff with syntax highlighting.

Polish & security:

• Default log level INFO (opt into DEBUG via codeHistorian.debug); stop popping
  the output panel on every settings save.
• Stop logging API keys / full settings payloads to the output channel.

Tests/docs:

• DB-layer integration tests against sql.js in Node (compression round-trip,
  BM25 ranking, bookmarks, retention, symbol/file lookup, stats) via a vscode
  stub. 46 tests total.
• CHANGELOG + README updated.