fix(transform): cap rendered image bytes per request

[mkhalid-s]

Closes #157.

What changed

• enforce an 18 MiB default decoded-image budget for Anthropic Messages requests
• count caller-supplied base64 images before admitting pxpipe PNG groups
• admit static slabs, tool results, and history atomically; keep a complete group as native text when it would cross the remaining budget
• preserve caller images unchanged and flag requests that are already over budget
• expose PXPIPE_MAX_IMAGE_BYTES consistently in Node and Workers
• persist generated bytes, caller-image bytes, skipped-group telemetry, and exact serialized request bytes
• warn in host logs and in the dashboard, including a near-budget warning at 90%

Validation

• 930 tests passed
• typecheck passed
• production build passed
• focused coverage includes exact byte boundaries, caller and nested images, static/tool/history fallback, invalid Worker configuration, tracker fields, and dashboard warnings

Scope

The limit follows the decoded image-byte metric behind the observed reliability cliff; exact serialized request size is reported separately. pxpipe does not delete caller images or automatically retry upstream failures without compression.