feat(runtime): gate node:dgram behind mod-dgram (binary size)

[proggeramlug]

What

First instance of a per-Node-module gating pattern: node:dgram (UDP sockets — crate::dgram + crate::dgram_reactor, ~43 KB incl. the js_dgram_* externs codegen emits direct calls to) is now compiled only when a program imports it.

Result

hello-world drops ~0.1 MB (dgram off). A real dgram program (createSocket/bind/send/on-message) auto-enables the feature, links + runs correctly. No speed change — pure cfg gate.

How

• mod-dgram cargo feature (in default) gates crate::dgram + crate::dgram_reactor, the ("dgram", …) dispatch arm, and the gc_init dgram-reactor root-scanner registration.
• The compiler detects module: "dgram" in the HIR (a dgram namespace can only arise from importing it) → ctx.uses_dgram → forwards perry-runtime/mod-dgram + cache-keys it. dgram is runtime-only, so it's not in native_module_imports (that set tracks only requires_stdlib modules) — HIR detection is the correct signal.
• codegen only emits js_dgram_* calls for dgram programs (which get the feature on), so the off build links cleanly with no dangling symbols (verified via auto-optimize link of both a dgram program and hello-world).

Stacked on #5159 (diagnostics gate); will retarget to main after that merges.

The big remaining lever (de-coupling console/process from the monolithic dispatch_native_module_method so all dispatch-only modules dead-strip, ~400 KB) is a deeper refactor for a separate, supervised PR.

[coderabbitai]

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