Add a SECURITY.md with a vulnerability disclosure policy

## Summary

The repository has no `SECURITY.md` and no documented process for reporting a vulnerability.

## Current state

The repo root contains `CODE_OF_CONDUCT.md` and `CONTRIBUTING.md`, but no `SECURITY.md`. GitHub's "Security" tab has no security policy configured, and there's no mention anywhere (README, CONTRIBUTING, wiki) of how a vulnerability should be reported, what response time to expect, or which versions receive fixes.

## Why this matters

Teams evaluating a dependency for use in a production environment routinely check for a published vulnerability-disclosure process as part of their due-diligence/vendor-review checklist. Without a `SECURITY.md`, there's no clear, private channel to report a potential issue (e.g. a memory-safety bug reachable from untrusted input in the DOT parser, or an algorithmic complexity issue that could be abused as a DoS vector) — reporters are left to use public issues, which is inappropriate for anything sensitive before a fix is available.

## Suggested resolution

- Add a `SECURITY.md` describing:
- Which versions/branches currently receive security fixes.
- A private reporting channel (e.g. GitHub's "Private vulnerability reporting" feature, or a maintainer email).
- Expected acknowledgement/response timelines.
- Enable GitHub's private vulnerability reporting for the repository.
- Link `SECURITY.md` from the README.

## Acceptance criteria

- [ ] `SECURITY.md` exists at the repo root and is picked up by GitHub's Security tab.
- [ ] A private reporting mechanism is enabled and documented.
- [ ] README links to the security policy.

Look at the existing CODE_OF_CONDUCT.md and CONTRIBUTING.md files in the repository root for formatting. Use GitHub's documentation on creating a SECURITY.md file and enabling private vulnerability reporting. The new file should be placed in the repo root and linked from the README. Check the repository's Security tab after creation to confirm it is recognized.

Written by the indexing model from the issue text.