Stub registry + first-call runtime diagnostic for no-op stubs

[proggeramlug]

Spun out of #455 (DX feedback from @justin0mcateer); companion to #463.

Problem

Some symbols exist in Perry's runtime but are intentional no-ops — for example the harmonyos perry_ui_* / perry_system_* / perry_updater_* direct-call stubs added in #399 / #477 / #513, or the cross-platform UI symbols that are stubbed on the wrong host. Compile-time erroring on these is wrong (the call is intentional and the platform-specific build will resolve to a real impl), but silently doing nothing at runtime is the same DX cliff #455 describes — the program runs and produces wrong output with no signal.

Proposal

A stub-registry layer in the runtime:

1. Manifest of stub symbols — assembled from direct_call_stubs in crates/perry-runtime/build.rs plus any other code paths that emit no-op fallbacks. Each entry knows the symbol name, the reason ("not implemented on macos build of harmonyos crate", etc.), and ideally a tracking issue.

2. First-call diagnostic — wrap each stub so that the first invocation prints once to stderr:

   [perry] warning: `notificationSchedule` is a no-op stub on this platform
                    (harmonyos-only; tracking: #399)
   

   Subsequent calls stay silent (or are gated behind PERRY_STUB_DIAG=verbose for the every-call mode).

3. perry check <entry.ts> subcommand — static scan of imports against the stub manifest. Surfaces the warnings before the user even runs the binary. Same machinery Compile-time error for unimplemented Node / Web APIs #463 uses for unimplemented-API resolution can power this.

Why a separate diagnostic from #463

#463 errors on symbols Perry doesn't have. This issue warns on symbols Perry has-but-stubs. Both flow from the same manifest infrastructure but target different audiences (compile-time hard fail vs. runtime soft warning).

Acceptance

• Calling a stubbed symbol prints exactly one warning per symbol per process.
• PERRY_STUB_DIAG=off silences; =verbose prints every call.
• perry check app.ts reports the same warnings statically.
• Stub manifest is derived from direct_call_stubs (no hand-maintenance).