fix(search): block IPv4-mapped and NAT64 SSRF literals

[thegeorgepu]

Summary

• classify canonical IPv4-mapped IPv6 literals before web_fetch
• classify NAT64 /96 embeddings, including single-group tails, without shifting the embedded IPv4 value
• fail closed for malformed recognized prefixes and reject out-of-range dotted octets
• register the focused SSRF suite in the canonical test harness, with a pin that requires one executable registration line

This is a narrow repair for finding 1 in #60. Redirect-per-hop and DNS-policy work remain outside this PR.

Evidence

• exact candidate: 8f8e321f4def104aac2d7dfec8679a4b5dd152e4
• tree: f77d74c89bae0dbcae473d821453a9108ede7af4
• focused production-import suite: 54/54 pass
• current main with the candidate tests: 25 intended failures
• original worker candidate with the expanded tests: 19 intended failures
• wrong single-group NAT64 mutation: 6 intended failures
• malformed recognized-prefix fail-open mutation: 12 intended failures
• test-registration removal/comment/substring mutations: all fail
• independent local review: GO on the exact clean head/tree

The broader package check could not be claimed locally because the borrowed dependency tree did not contain the repository's tsgo/Vitest type dependencies. This draft intentionally relies on the repository's natural CI for the full dependency-backed check before promotion.

Review checklist

• Negative cases reach the intended address-classification mechanism rather than passing through an earlier refusal.
• Positive public IPv4/IPv6 controls remain reachable.
• Tests import the production extension and are wired into the installed canonical harness.
• Missing, malformed, range-invalid, mapped, and NAT64 variants are covered.
• Old behavior and targeted defect restorations fail the intended assertions.

Closes the IPv4-mapped/NAT64 portion of #60; does not close the redirect finding.