Fix 3x scan performance regression caused by WatchesFile recreating watchers on every open()

[Copilot]

Scans (forward, backward, join, range) were ~3x slower than v0.7.2 because ReadOnlyPartition (via the WatchesFile mixin) was creating and destroying an OS-level file watcher on every partition.open() call — which fires once per document read during a scan via getPartition().

Root cause

WatchesFile.open() unconditionally called watchFile() after delegating to super.open(), but ReadablePartition.open() returns true both on first open and when already open (if (this.fd) return true). So each of the 10,000 getPartition() calls per scan triggered a full watcher teardown + recreation (~3.5 µs vs ~0.02 µs for a null check).

// Before — watchFile() fires on every call, even when already open
open() {
    if (super.open()) {
        this.watchFile();   // ← recreates Watcher every time
        return true;
    }
    return false;
}

// After — only create the watcher once
open() {
    if (super.open()) {
        if (!this.watcher) {
            this.watchFile();
        }
        return true;
    }
    return false;
}

Changes

• src/WatchesFile.js: Guard watchFile() with if (!this.watcher) so the file watcher is created only on the first open(), not on subsequent no-op calls.
• src/Storage/ReadableStorage.js: Guard this.emit('preRead', …) with listenerCount('preRead') > 0 to skip EventEmitter overhead when no hooks are registered.
• src/Storage/WritableStorage.js: Same guard for this.emit('preCommit', …).

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[coveralls]

coverage: 97.892% (+0.009%) from 97.883%
when pulling 7e68ccd on copilot/bisect-performance-regression
into eaa8ee1 on main.