[P2][testing] add HTTP/DSL/plugin/libs regression matrix #58

Summary

H-awk now spans HTTP runtime, DSL desugaring/type checks, plugin hooks, optional Zig libs, cache, supervisor, and proc/message experiments. The test suite should encode a matrix that prevents regressions across those responsibility boundaries.

Proposed test matrix

HTTP/runtime

• GET/POST/DELETE basic route dispatch.
• 404/405/Allow.
• HEAD/OPTIONS behavior.
• Header/body size limits.
• /inet/tcp/ fallback and libs/net path when available.

DSL

• let, ??, |>, dot dispatch.
• when...of result/option branches.
• typed ng and union exhaustiveness.
• safe HTML type checks.
• useful error messages and line numbers.

Plugin

• discovery, disabled plugin, missing manifest/hook.
• init, pre_request, post_request, shutdown.

Libs/cache/proc

• native lib present/missing behavior.
• cache memory/file/zig backends.
• supervisor restart intensity.
• mailbox/objectspace single-worker vs multi-worker behavior.

Acceptance criteria

• CI runs fast unit tests on every push.
• Slower e2e/native tests are separated behind a make target or CI job.
• Every P0/P1 issue from this review has at least one regression test target.

Priority: P2
Area: testing/ci
Type: feature