feat(tls): add opt-in ex_ssl 0.4 with verified TLS 1.2 and 1.3

[gsmlg]

Behavior

Add explicitly selectable :ex_ssl TLS to Fetch, HTTP/1.1, HTTP/2, WebSocket and EventSource. OTP :ssl remains the default. Backend selection survives redirects and reconnects, with no automatic fallback or replay of uncertain request bytes. HTTP/3 and WebTransport retain their QUIC implementation.

Use published ex_ssl 0.4.0 (~> 0.4.0, locked to 0.4.0). The adapter supports verified TLS 1.3 and explicitly configured TLS 1.2, validated client identities, ordered algorithm preferences, and an explicit TCP-option allowlist. Client identities stay bound to their original redirect origin. TLS 1.3 automatic ticket resumption is opt-in and preserves trust/profile/ALPN isolation. Unsupported options fail explicitly.

Correctness and packaging

Preserve complete HTTP/2 early responses when optional control writes encounter authenticated peer closure, including responses spanning TLS records and post-completion NO_ERROR resets. Success still requires END_STREAM, complete headers, valid Content-Length and bounded framing. Stop pending upload DATA at response completion. Truncation, required-write failures, cancellation and deadlines remain errors.

Run child app suites through the umbrella root and verify all five built packages in a fresh external consumer with transitive dependency resolution. Deterministic regressions use explicit peer/message barriers. TLS 1.2 large-response tests hold the reference peer until the entire response is read; separate Phase 0 tests continue to exercise close-before-control-write behavior.

Validation

• Merged tree: 444 tests + 20 doctests, zero failures.
• Formatting, development/test strict compilation and Credo pass. Dialyzer passes with four existing ignored diagnostics and no new diagnostics.
• Packaged-source integration: 47 tests, zero failures after fixing a test-peer shutdown race; the initial intermittent failure is recorded in the progress ledger.
• Published Hex ex_ssl 0.4.0: all 47 packaged TLS feature tests and all 11 deterministic cross-record cases pass. The separate cold consumer smoke passes with transitive-only resolution and verified package metadata.
• GitHub checks below validate the final pushed revision; local commands and corrected historical failures are recorded in docs/pr-14-validation.md.

Compatibility boundaries and historical evidence are documented in docs/ex-ssl-consumer-contract.md, docs/pr-14-validation.md, and the ex_ssl cross-repository progress ledger. This does not claim full OTP parity or a completed human security review. TLS 1.2 resumption, mTLS resumption and 0-RTT remain unsupported.