Browser-compatible transport seam for ClientRuntime

[gmpassos]

Summary

Makes the OmnyShell client compile to JavaScript and run in a browser, so a pure web client (omnyshell_web) can reuse ClientRuntime over the platform WebSocket instead of reimplementing the protocol. Native CLI / Hub / Node behaviour is unchanged.

Changes

• Transport seam: ClientConfig.connectionFactory (a ConnectionFactory), resolved via a conditional import that selects a dart:io wss:// socket on the VM and the browser WebSocket (WsChannelConnection) under dart2js.
• TLS knobs relocated: ioConnectionFactory({securityContext, onBadCertificate, pingInterval}), exported from omnyshell_client.dart, builds the native factory. The CLI's --ca / --insecure-skip-verify behaviour is unchanged.
• dart:io isolated from the client graph: PlatformInfo.local() moved to a dart:io-only extension (platform_info_io.dart); the local-tunnel TCP bridge sits behind a conditional-import factory; PlatformInfo is now dart:io-free.
• Drop detection: ClientConfig.onDisconnected.
• Browser barrel: package:omnyshell/omnyshell_client_web.dart exports only the JS-compatible subset.
• Embedded example updated; bumped to 1.29.0 with CHANGELOG.

Breaking change

ClientConfig no longer accepts securityContext / onBadCertificate (those dart:io types blocked browser compilation). Native callers migrate to:

ClientConfig(
  hubUri: uri,
  credentials: creds,
  connectionFactory: ioConnectionFactory(
    securityContext: ...,
    onBadCertificate: ...,
  ),
)

HubConfig and NodeConfig are unaffected.

Verification

• dart analyze . — clean.
• dart test — all 471 tests pass.
• dart compile js of an entrypoint importing ClientRuntime produces no "dart:io not supported" errors.

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