ci: add non-blocking big-endian portability smoke under emulation

[la3lma]

Closes #46.

Summary

• execute a bounded semantic suite on s390x-unknown-linux-gnu under QEMU instead of stopping at cross-compilation
• pin cross 0.2.5 and the s390x container image by immutable digest
• retain target/endianness, emulation-host identity, tool, image, and test logs
• cover raw UTF-16/surrogates, predicates, case folding, exact spans/digest, callback panic recovery, lifecycle reuse, portable-SIMD refusal, and a byte-order-corruption mutation
• keep the preview lane nonblocking, non-native, and explicitly non-performance
• document one-command Linux reproduction and generate its HTML mirror

Local verification

• cargo fmt --check
• strict workspace clippy for all targets/features
• cargo xtask ci
• focused case-fold, big-endian harness, and host-identity contract tests
• actionlint
• bash -n scripts/big-endian-smoke.sh
• optimization-ledger render and verification
• git diff --check

The one-command emulated execution requires a Linux Docker/binfmt/QEMU host; macOS validation therefore verifies the frozen contract and leaves executable proof to this PR's hosted Ubuntu lane. The PR remains draft until that lane itself passes and its retained evidence has been reviewed.

[la3lma]

Hosted acceptance evidence

The final head 0fc2dcd3385f97b4c0176c8fedea44653940718d is green across every required and preview job.

The decisive nonblocking lane passed in release-readiness run 31334739294, job 93298621045, in 1m20s.

The earlier retained evidence run 31334450537 was downloaded and reviewed before being bound into the repository record:

• host identity says execution_mode: emulated, host x86_64-unknown-linux-gnu, declared target s390x-unknown-linux-gnu
• executed binary reports target_arch=s390x target_endian=big target_os=linux
• exact pinned image digest resolved: sha256:0d8edc92c39158abe211fddfc9617bf5b7865c2ae0f58873f9cf0c58bb315271
• bounded semantic tests: 2 passed, 0 failed
• portable-SIMD refusal test: 1 passed, 0 failed
• retained artifact SHA-256: 1dfdca69494ddfbc7b74c1b8c21fcd510d4f3162ee0e8de29a9304982d5c7680
• retained test-log SHA-256: 5c551fe06f0244ef1823bf80c9a368b90451038a5c16375a354a745cb501c3ed

Local cargo xtask ci, HTML-universe freshness, host-identity contracts, actionlint, shell syntax, and diff checks also passed. This satisfies #46 without claiming native support or performance.