Implement the NSGI bridge cdylib with an embedded CRuby VM

[himura467]

Summary

First working slice of the NSGI-compliant Ruby bridge, covering the core of #1, #2, and #3, with initial verification tooling for #6.

• ext/nsgi: cdylib exporting nsgi_handle/nsgi_free_response. Boots an embedded CRuby VM on a dedicated thread using the canonical embedding sequence (ruby_init_stack + ruby_setup + ruby_options("-e", "0") + ruby_exec_node; bare ruby_setup() leaves Ruby-defined core methods undefined, see the note on Set up the gem skeleton and boot an embedded CRuby VM inside the NSGI cdylib #1). Requests from foreign host threads are handed off via an MPSC queue with per-job condvars.
• Zero-copy request path: request fields reach the app as read-only external IO::Buffer views (rb_io_buffer_new with EXTERNAL|READONLY); every buffer is tracked and force-invalidated when the request completes, on both success and raise paths.
• Response path: each response is serialized into a single app-owned malloc block ([NsgiHeader array][header bytes][body bytes]) whose base is recoverable from the struct itself, so nsgi_free_response frees it from any thread with zero VM interaction. A static, never-freed 500 is the last resort.
• Fault containment: app exceptions become 500s in NSGI.__dispatch; bridge-level raises are caught by rb_protect; Rust panics by catch_unwind. Nothing unwinds across the ABI.
• lib/: pure-Ruby side (NSGI::Request, NSGI.__dispatch); host/: mock NSGI host + test fixture; examples/hello.rb: minimal app.

One upstream gap discovered during buffer-lifetime testing was filed separately as #8 (freed IO::Buffer reads as empty instead of raising).

Verification

cargo build
NSGI_APP=host/fixture.rb NSGI_LIB=lib cargo run -p nsgi-mock-host
  => mock host OK: 204 requests (8 threads x 25 + warmup, error and stash paths)
NSGI_APP=host/fixture.rb NSGI_LIB=lib leaks --atExit -- ./target/debug/nsgi-mock-host
  => 0 leaks for 0 total leaked bytes

The mock host dlopens the cdylib as a real host would and asserts: correct echo responses under 8-thread concurrency, app exception surfacing as a 500 (never a crash), a stashed buffer being unreadable after its request, and exactly-once nsgi_free_response on every path.

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