[Web] Retain evicted shape tuples until teardown

[atirna]

Summary

• retain ShapeTuples evicted from the bounded LRU until CacheState.dispose()
• dispose each retained or still-cached tuple exactly once at TVM-instance teardown
• add a regression test for the live borrowed-reference lifecycle

This prevents an LRU miss from disposing a tuple which a pending GPU dispatch may still use.

Validation

• npx jest --config jest.config.js --runTestsByPath tests/node/test_cache_state.js --runInBand
• npx eslint -c .eslintrc.cjs src/cache_state.ts tests/node/test_cache_state.js
• npx tsc --noEmit -p tsconfig.json

The full npm test requires generated dist/ WASM artifacts not present in the sparse checkout; the existing integration suites cannot resolve ../../dist in that environment.

Relates to mlc-ai/web-llm#844.