Rename `fps` to `fpsMonitor` and `.fps()` to `.value()`

[vs-borodin]

Which @signality/* package(s) are relevant/related to the feature request?

core

Description

fps() is named after the metric, but it returns a controller (isRunning / start() / stop()), not a value.

The result is a stutter at every call site: fpsMonitor.fps(). Other core utilities that carry a single primary value expose it as value (cva, queryParams), so fps is also inconsistent with the rest of the API.

Proposed solution

Rename to match how the utility is actually used, with a temporary deprecation window instead of a hard break.

New API:

import { fpsMonitor } from '@signality/core';

readonly monitor = fpsMonitor();
// monitor.value()      (was monitor.fps())
// monitor.isRunning()
// monitor.start() / monitor.stop()

Renames:

Before	After
fps()	fpsMonitor()
FpsRef.fps	FpsMonitorRef.value
FpsOptions / FpsRef	FpsMonitorOptions / FpsMonitorRef
FPS token	FPS_MONITOR
@signality/core/browser/fps	@signality/core/browser/fps-monitor
docs /browser/fps	docs /browser/fps-monitor

Alternatives considered