perf(hmr): avoid eager subChain allocation in propagateUpdate

[shulaoda]

In propagateUpdate, subChain was allocated at the top of the importer loop for every importer, but it is only used in two branches.

Problem

• const subChain = currentChain.concat(importer) runs for every importer edge during HMR propagation.
• It is only consumed by the acceptedHmrDeps branch and the recursive call. The acceptedHmrExports branch continues without ever using it — so for modules accepted via export bindings (e.g. barrel/re-export files with many importers), each allocation is immediately discarded.

Fix

• Inline currentChain.concat(importer) at the two sites that actually use it, so the export-accepting continue path allocates nothing. The two sites are mutually exclusive, so this computes the chain at most once per importer. No behavior change.

Tests

• No test is included: this is a behavior-preserving change that only defers an allocation, so there is no observable behavior difference to assert (a fails-without/passes-with test isn't possible). propagateUpdate correctness remains covered by the existing HMR tests and the playground/hmr e2e suite.