probe-image-size: Quadratic-time Denial of Service in the SVG Parser

## Overview `probe-image-size` scans the SVG header with a searching regular expression, `/<[-_.:a-zA-Z0-9][^>]*>/`. On input that contains many `<` characters but no `>`, the engine restarts the `[^>]*` scan at every `<` position and runs to end of input each time, giving quadratic time complexity. Both the synchronous and the streaming parser are affected. ## Impact Every entry point that reaches the SVG parser is affected: `probe.sync()`, `probe(stream)` and `probe(url)`. The URL form is the most exposed one — the input is fetched from a remote host, so an attacker only needs to supply a link. Processing a crafted buffer blocks the Node.js event loop at 100% CPU for the whole duration. In production environments such as upload validators, image proxies or link unfurl services, a small number of concurrent requests is enough to deny service. ## Root Cause Analysis Two independent problems. 1. **Absence of input size cap in the sync path.** `lib/parse_sync/svg.js` copied the entire buffer into a string and matched against it. There was no size limit at all, so cost scaled with the size of the attacker-supplied buffer. 2. **Repeated rescanning in the stream path.** `lib/parse_stream/svg.js` did cap accumulated data at 64 KB, but called `parseSvg(str)` on the whole accumulated string on *every* chunk, giving `O(chunks × N²)`. The cap does not help here: the more chunks the input is split into, the more times the quadratic scan is repeated. Chunk size is influenced by the sende