[labs/virtualizer] Add permanent, toggleable lifecycle logging

### Description

Add permanent diagnostic logging to the virtualizer's key lifecycle points, controlled by a runtime or build-time toggle. The logging should cover the full update pipeline and be filterable by subsystem.

### Motivation

During debugging of #4693 and #5006, lifecycle logging at specific points in the virtualizer pipeline was essential for understanding scroll behavior, position estimation, freeze/unfreeze cycles, and DOM update timing. Currently this logging must be manually added and removed each session.

### Logging points

The following lifecycle points should be instrumented:

**Virtualizer.ts:**
- `_updateDOM` — range, child positions, scroll error, scrollTop
- `_finishDOMUpdate` — positions flushed to DOM with first/last item positions
- `_handleScrollEvent` — freeze detection (delta, threshold)
- `_unfreezeScroll` — freeze release with scrollTop

**FlowLayout (flow.ts):**
- `_updateLayout` — cross-axis dimension changes, cache clearing
- `_updateVirtualizerSize` — scroll correction firing (size ratio, scroll position scaling)

### Requirements

- Zero cost in production builds (depends on #5281)
- Togglable at runtime for development builds
- Compact, single-line format with `[virt:xxx]` prefix for console filtering
- Key metrics (range, positions, scrollTop, scrollError) in every relevant log

### Depends on

- #5281 (build support for dev-only code blocks)

Start by reviewing the dev-only build support in #5281, then inspect the lifecycle methods listed in Virtualizer.ts and flow.ts. Trace the update pipeline and identify how runtime toggling and subsystem filtering should work. Done means development builds provide compact [virt:xxx] logs at all specified points while production builds incur zero cost.

Written by the indexing model from the issue text.