fix(http): server.ref()/unref() return this instead of a raw number (#5011)

[proggeramlug]

Summary

Fixes #5011 — server.ref() and server.unref() on a node:http/node:https server returned the receiver handle as a raw number instead of the server object. Node returns this, so http.createServer(cb).unref().listen(...) broke under Perry with TypeError: (number).listen is not a function (test-http-request-method-delete-payload.js and friends).

const http = require('http');
const s = http.createServer(() => {});
console.log(typeof s.unref(), s.unref() === s);
// Node:  object true
// Perry (before): number false
// Perry (after):  object true

Root cause

crates/perry-codegen/src/lower_call/native_table/http_server.rs had entries for listen/close/closeAllConnections/… but none for ref/unref, so they fell through to a generic handler that returned the handle as a number — the exact shape listen had before #2129.

Fix

• native table: added ref/unref NativeModSig rows (ret: NR_PTR) for both the HttpServer and HttpsServer class filters.
• runtime: new js_node_http_server_ref/_unref and js_node_https_server_ref/_unref return the receiver handle (so s.ref() === s) and toggle a new refed flag on HttpServer. server_is_active now ignores a listening-but-unref()ed server, so the event loop can exit while the server is still bound (Node semantics) — pending listen callbacks and queued requests still keep the loop alive long enough to flush in-flight work.
• dynamic dispatch: mirrored ref/unref in the server: any dispatcher (handle_dispatch.rs).
• wiring: registered the externs in ext_registry.rs, runtime_decls/stdlib_ffi.rs, and the force_link_http_server anchor table; added ref/unref manifest rows and regenerated docs/src/api/reference.md.

Acceptance — verified locally

• typeof s.unref() → object, s.unref() === s → true
• s.ref() === s → true
• createServer(cb).unref().listen(...) works (listening on number)
• A listening unref'd server lets the process exit (exit 0, no hang)
• cargo test -p perry-api-manifest → 32 + 4 passed, 0 failed (manifest/docs drift clean)

Repro added at test-files/test_http_server_unref_5011.ts.

Not in scope

Separate (number).<method> cases tracked under #4975: new http.Agent().createConnection(...) returning a number (test-http-socket-encoding-error) and server.ALPNProtocols not being a Buffer (test-https-argument-of-creating).