refactor(server): index imported modules by url in soft invalidation

[shulaoda]

handleModuleSoftInvalidation rewrites import timestamps by scanning the whole mod.importedModules set for every parsed import, i.e. O(imports × importedModules).

Change

• Build a Map<url, EnvironmentModuleNode> from mod.importedModules once, then look up each import's hmrUrl in O(1). The map keeps the first node per url, matching the previous break-on-first-match behavior.

This is a readability / scalability cleanup (linear instead of quadratic for modules with many imports, e.g. barrels). Behavior is unchanged.

Tests

• No fails-without/passes-with test is included: this is a behavior-preserving change with identical output, so there is nothing new to assert. Soft-invalidation behavior is exercised by the playground/hmr e2e suite, and the existing transformRequest unit tests pass.