feat: gate releases on verification + test-parity fixes

[abienkowski]

Gates the release pipeline on full verification before tagging artifacts, closes the test-parity gaps found while auditing coverage, and enforces the Unix-socket security boundary across the sock integration suite.

Part 1 — Release verification gate

• Makefile: backend-parameterized verify (BACKEND=), new ci-verify (Quint TS backend) and release-verify (Quint Rust backend) aggregate targets
• CI: runs on pull requests only (drops duplicate push-to-main runs); jobs reuse Make targets
• Release: new release-verify gate job blocking tag creation; version job creates a draft prerelease; new publish-release job flips draft → published only after all three artifact jobs succeed
• docs/release.md: documents local gates

Closes part of #6 and #8.

Part 2 — Test-parity fixes (audit follow-up)

Go

• Image prefix matching respects namespace boundaries — chainsafe/lodestar no longer matches evilfoo-lodestar or lodestar-backup (manager.go)
• New readonly + validation-path middleware tests (chain_test.go)

Rust

• Forwarded Content-Length is rewritten to match the (possibly mutated) body length — fixes daemon-side JSON truncation (handler.rs)

TypeScript

• Flags honor the --flag=value form (new flags.ts module + tests)
• Proxy actually listens on the parsed TCP host/port — the old string overload was a no-op (index.ts)
• Content-Length derived from the bytes actually forwarded (transport.ts)
• Handler tests ported to match Go/Rust (handler.test.ts)

Integration / CI

• Compose build context + project name parameterized by IMPL (go/rs/ts)
• --build on integration runs; new test-integration-rs / test-integration-ts targets
• CI runs all three integration suites
• Numeric user: "0:0" for scratch-based Rust/TS images

Spec & config

• Route POST /containers/:name/pause to allowAction; README documents 9 invariants (6 P0 / 3 P1)
• Beacon sample policy pins the host systemd account to uid/gid 100 (system user), independent of the in-container user

Part 3 — Unix-socket security boundary (403 on socket permission denial)

The group-restricted Docker Unix socket is the security model; connecting under a group that lacks access must surface as 403 Forbidden, indistinguishable from a middleware policy denial:

• Go: ReverseProxy ErrorHandler maps EACCES/EPERM on the socket dial → 403 (transport.go); tests for 502 (missing socket), 403 (permission), and errno unwrapping
• Rust: new TransportError::SocketPermission variant → 403 in the handler; transport + handler tests for both 403 and 502 paths
• TS: Transport emits 403 on EACCES/EPERM (transport.ts, isPermissionDenied helper)
• TS --docker-host: accepts a Unix socket path only — rejects tcp:///http:///unix:// schemes so all three implementations run the sock suite over a real socket
• Makefile: test-integration-sock parameterized by IMPL with --build + -rs/-ts variants
• Removed docker-compose.tcp.yml + test-integration-tcp: TCP forwarding bypasses user/group socket ownership, and the dial path was already unix-only (broken) for Go/Rust
• Audit-log tests in all three languages (temp-file JSON round-trip for allow/deny)

Verification

• Go: 74 unit tests + go vet clean
• Rust: 112 unit tests + cargo check clean
• TS: 108 unit tests + tsc clean
• Integration: 26/26 (go, rs) via compose + ts 26/26 natively with the new --docker-host flag
• Sock suite: 8/8 for go and rs — denied proxy returns 403 on every route
• Quint: typecheck + 9-invariant simulation with no violations (unchanged)