dist: publish to opm.openresty.org and add Windows CI

[membphis]

Motivation

Two ecosystem expansion items that increase reach without changing core functionality:

1. OpenResty Package Manager (OPM)

lua-cjson is bundled with OpenResty and available on opm.openresty.org. qjson's primary benchmark target is OpenResty (the bench harness uses resty). Publishing to OPM lets OpenResty users install via:

opm get api7/lua-qjson

OPM requires an opm.json manifest. The main challenge: OPM packages are expected to be pure-Lua or include pre-built C modules. The Rust compilation step needs to either:

• Complete during opm install (requires Rust on the server, same barrier as LuaRocks today), or
• Ship a pre-built .so (depends on Issue docs: production deployment guide for build/runtime separation (no Rust at runtime) #68 landing first).

Suggested order: land pre-built binaries (Issue #68) first, then add OPM manifest pointing to the same assets.

2. Windows CI

The rockspec explicitly lists supported_platforms = { "linux", "macosx" }. lua-cjson runs on Windows (AppVeyor CI). qjson uses cdylib + dlopen via LuaJIT FFI — dlopen is unavailable on Windows; the equivalent is LoadLibrary.

Adding Windows support requires:

• Verifying ffi.load("qjson") works on Windows LuaJIT (it uses LoadLibrary under the hood — LuaJIT FFI handles this transparently).
• Building libqjson.dll in CI (cross-compile from Linux or native Windows runner).
• Adding windows to supported_platforms in the rockspec.

This is lower priority than the other distribution items but would close the gap with lua-cjson's platform matrix.

Affected files

• opm.json (new)
• .github/workflows/ci.yml — add Windows job
• rockspec/lua-qjson-*.rockspec — add windows to supported_platforms

[coderabbitai]

🔗 Related PRs

#43 - test: add cjson and simdjson fixture coverage [merged]
#45 - feat: release v0.1.0 [merged]
#51 - perf: skip redundant validation in eager decode, inline depth check [merged]
#54 - perf: Lua encode + cache + scanner + validation stack optimizations [merged]
#58 - docs: fix install instructions and qualify ARM64 performance claims [merged]

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[membphis]

Closing this combined tracking issue. OPM publishing should be revisited after pre-built binary distribution is decided, and Windows support is a separate platform-support scope rather than something to develop from this bundled issue.