Limit playground builds to requested platform dependencies

[gioneill]

Summary

Problem this solves: don't build use platform level stuff that I'm not trying to build (Android NDK when building Mac OS app, example)

• Scope Valdi workspace initialization to the selected platform by wiring platform_dependency_rule and VALDI_PLATFORM_DEPENDENCIES into WORKSPACE.
• Use platform Bazel configs (--config=ios|android|macos|web) and switch playground scripts to those configs.
• Prevent non-Android builds from resolving Android toolchain dependencies indirectly.

Changes

• WORKSPACE
  • Add platform_dependency_rule and VALDI_PLATFORM_DEPENDENCIES.
  • Call valdi_initialize_workspace(VALDI_PLATFORM_DEPENDENCIES).
• .bazelrc
  • Add build:ios, build:android, build:macos, build:web repo env mappings.
• Playground scripts
  • scripts/bazel_ios_install.sh
  • scripts/bazel_android_install.sh
  • scripts/bazel_macos_run.sh
  • scripts/bazel_web_serve.sh

Verification

• bazel build --config=macos --nobuild //valdi_modules/playground:app_macos
• bazel build --config=web //valdi_modules/playground:playground_export_npm

[gioneill]

Valdi_Widgets is where this surfaced, but the interesting part is that Valdi already seems to provide the intended pattern for handling this. In Valdi’s own WORKSPACE, the repo does not call valdi_initialize_workspace() bare; it first creates platform_check, loads VALDI_PLATFORM_DEPENDENCIES, and then passes that through to valdi_initialize_workspace(...) from workspace_init.bzl. By contrast, the old Valdi_Widgets setup was effectively:

load("@valdi//bzl:workspace_init.bzl", "valdi_initialize_workspace")

valdi_initialize_workspace()

whereas Valdi’s own repo uses the more explicit pattern:

load("//bzl:workspace_init.bzl", "platform_dependency_rule", "valdi_initialize_workspace")

platform_dependency_rule(name = "platform_check")
load("@platform_check//:target_platform.bzl", "VALDI_PLATFORM_DEPENDENCIES")

valdi_initialize_workspace(VALDI_PLATFORM_DEPENDENCIES)

So my read is not necessarily that Valdi needs new semantics here, but that downstream repos like Valdi_Widgets should follow the same platform-scoped initialization pattern that Valdi itself already uses. This PR does that, and it fixes the case where a non-Android build like //valdi_modules/playground:app_macos can otherwise end up pulling Android workspace dependencies unnecessarily.