perf: avoid cloning env during expansion

[chenjiahan]

Performance issue

expandValue currently creates a merged env object for every parsed .env key:

const env = { ...runningParsed, ...processEnv }

This means each key expansion clones the full processEnv object. In large projects, processEnv can contain thousands of keys, so loading a .env file with many entries repeatedly pays this cloning cost.

Optimization

This PR avoids cloning the full env object during each expandValue call.

Instead, it looks up env values lazily only when an expansion expression references a key.

Benchmark

Local benchmark with:

• 2,000 processEnv keys
• 200 parsed .env keys
• 10 runs

Version	Avg	Min	Max
Current	50.581 ms	49.217 ms	54.207 ms
Optimized	0.101 ms	0.061 ms	0.269 ms

Average speedup: ~498.6x.

[motdotla]

hi @chenjiahan - thank you for this. i've moved internals to use the new smarter @dotenvx/primitives though..

new major release coming out and then can reconsider a PR like this.