feat(net): SSRF-safe fetch primitive + DNS-pinned rawMcpProbe

[bokelley]

Closes #573. First of three PRs bundling the verifier follow-ups from the RFC 9421 review (#583 items 1+2, #584, #585 remain for PR B/C).

Summary

• Extract the DNS-pinning + SSRF-validation guard previously local to src/lib/testing/storyboard/probes.ts::fetchProbe into a reusable @adcp/client/net primitive. Future HTTPS fetchers (JWKS auto-refresh, revocation-list polling in signing: JWKS auto-refresh + revocation-list polling for long-running servers #584) consume the same guard.
• Rewire rawMcpProbe — previously using the node global fetch with no SSRF check — to dispatch through the primitive. Closes the TOCTOU gap where an attacker's authoritative nameserver could serve a public IP to a validator and a private IP to the actual connect.

The primitive

ssrfSafeFetch(url, options) returns a buffered { status, headers, body, pinnedAddress, pinnedFamily } or throws a typed SsrfRefusedError with one of: invalid_url, scheme_not_allowed, non_https_without_opt_in, dns_lookup_failed, dns_empty, always_blocked_address, private_address, body_exceeds_limit.

Guarantees (behavior preserved from existing fetchProbe, now uniformly applied):

• Scheme gate: https: only; http: under allowPrivateIp dev opt-in. file:/data:/ftp: always refused.
• DNS pinning: resolves A + AAAA once, validates the full set, pins the first validated address into undici.Agent.connect.lookup. Connect-time DNS is bypassed entirely — no rebind window.
• IMDS always refused: 169.254.0.0/16 + fe80::/10 refused even under allowPrivateIp.
• Redirects not followed (redirect: 'manual'). 3xx responses are returned with Location for inspection; callers MUST re-invoke ssrfSafeFetch with the new URL to re-validate — documented on the module contract.
• Body cap: 64 KiB default, throws body_exceeds_limit if exceeded.
• Per-request dispatcher: new Agent per call, closed in finally. Two concurrent calls with different pinned IPs don't interfere.

Address classification hardening (from security review)

Addressed two bypasses the reviewer caught:

• IPv6 zone IDs stripped before classification. fe80::1%eth0 now correctly flags as link-local. Previously a bracketed URL literal http://[fe80::1%eth0]/ under allowPrivateIp: true could reach IMDS.
• Non-canonical IPv4-mapped IPv6 normalized via net.BlockList instead of regex matching. 0:0:0:0:0:ffff:127.0.0.1 now classifies as loopback (previous regex required ::ffff: prefix).
• NAT64 (64:ff9b::/96) and 6to4 (2002::/16) wrapper prefixes refused unconditionally when allowPrivateIp is off — defense in depth against caller-edge translation into private targets.
• URL brackets stripped before DNS lookup and classification. https://[::1]/ now works correctly under allowPrivateIp and is rejected as loopback by default.

Info-disclosure hygiene

SsrfRefusedError.message carries the counterparty-supplied hostname only; the resolved IP lives on .address. A hostname that resolves into the caller's internal network now won't leak topology into compliance reports / log aggregators.

Changes in tree

• NEW: src/lib/net/{address-guards,ssrf-fetch,index}.ts
• MODIFIED: src/lib/testing/storyboard/probes.ts (−160 lines; fetchProbe and rawMcpProbe are now thin wrappers over the primitive)
• MODIFIED: src/lib/testing/storyboard/runner.ts (thread allow_http through to rawMcpProbe)
• NEW TEST: test/lib/net-ssrf-fetch.test.js (26 tests)
• MODIFIED TEST: test/lib/storyboard-security.test.js (updated 3 localhost tests to opt into allowPrivateIp; added 3 regression tests for rawMcpProbe should dispatch through DNS-pinned undici Agent (close TOCTOU gap) #573)
• NEW: .changeset/ssrf-safe-fetch-primitive.md (patch bump — rawMcpProbe is not in the public barrel; the tightened default is the point of rawMcpProbe should dispatch through DNS-pinned undici Agent (close TOCTOU gap) #573)

Test plan

• npm run typecheck clean
• npm run format:check clean
• npm run build clean
• node --test test/lib/net-ssrf-fetch.test.js — 26/26 pass (all new)
• node --test test/lib/storyboard-security.test.js — 68/68 pass
• npm test — 3671/3701 pass. The 12 failures are pre-existing Governance E2E suites broken by v3.0.0-rc.4: migrate to reallocation_threshold + human_review_required (breaking) #576's reallocation_threshold migration (verified they fail identically on clean main: 7 pass / 20 fail on governance-e2e.test.js alone).
• CI green (in progress)

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