Persist tree zoom, per-tab tree cache, keep expansion on refresh

[leoshone]

Three usability fixes for the JSON tree panel.

R1 — Tree zoom is persisted
The sidebar zoom (80%–250%, slider / Ctrl+wheel / double-click reset) now writes
[Others] TREE_ZOOM to JSONViewer.ini and is re-applied on start-up. The
percentage is stored rather than a concrete font size, so the setting survives
DPI changes. Zoom stays a panel-global property, not per tab.

R2 — Nothing is parsed automatically; tree state is cached per tab
Previously the panel either re-parsed on every tab switch (with "Follow current
tab" on) or went stale (with it off). Now:

• Starting Notepad++, switching tabs and opening files never read the document.
• Only Refresh JSON Tree parses and draws.
• Each buffer keeps an in-memory snapshot — node contents, expansion state and
  the selected node — so switching away and back restores the tree exactly as it
  was left, without re-parsing or redrawing.
• Tabs that were never refreshed show the empty placeholder root (JSON).
• Snapshots are dropped when a tab is closed and never survive a restart.

R3 — Refresh keeps the expansion state
Refresh rebuilds the tree and then restores the expansion/collapse state and the
selected node by matching node paths; paths that no longer exist are discarded.

Clean-ups

• Removed the now unused "Follow current tab" setting (UI, INI key and the
  NPPN_READY auto-refresh workaround it needed).
• Auto-format moved to NPPN_FILEOPENED, so merely switching tabs no longer
  rewrites a document. When it fires it formats the document only — the tree is
  still left alone until the user refreshes.

Testing

• gtest: 161/161 pass (TreeStateTest is new, ProfileTest extended).
• End-to-end inside a real Notepad++ instance: 19/19 assertions pass, covering
  no-auto-parse, per-tab snapshot isolation (including two levels of expansion),
  refresh preserving expansion + selection, snapshot dropped on tab close, and
  the zoom round-trip through JSONViewer.ini and a restart.

Note: this branch is on my fork for CI artifacts. Happy to rework anything
before it goes upstream.