Add browser-probe checkpoint and repeated-run leak hooks

[chubes4]

Problem

A single final browser snapshot is not enough to find memory spikes or leaks in browser-hosted Codebox flows. Consumers need named checkpoints across a user journey and repeated iterations to detect retained memory.

Proposed behavior

Add a generic checkpoint protocol to wordpress.browser-probe.

Possible contract:

• expose window.__wpCodeboxProbeCheckpoint(name, metadata = {}) inside the page before the probe script runs
• each call records a generic memory/performance snapshot
• write newline-delimited checkpoint records to files/browser/checkpoints.jsonl
• include checkpoint summaries in files/browser/summary.json

Add optional repeat/leak controls:

• repeat=<n> to run the probe journey more than once where feasible
• reset-between=none|reload|new-page to define repeated-run behavior
• summarize per-iteration peak/final/delta values

Acceptance criteria

• Probe scripts can record named checkpoints without app-specific code in WP Codebox.
• Checkpoint artifacts include timestamp, name, optional metadata, and generic memory/perf snapshot fields.
• Repeated-run output distinguishes iterations and final retained values.
• Existing browser probes are unchanged unless checkpoint/repeat options are used.
• Smoke coverage exercises at least two checkpoints and verifies checkpoints.jsonl plus summary references.