fix: honour READ_ONCE_TTL=0 instead of falling back to the default

[joyheroes]

Summary

Preserve READ_ONCE_TTL=0 as a valid configured value instead of treating it as a missing value and falling back to the default TTL.

Testing

• added regression coverage for zero TTL
• relevant tests pass

Closes #286

[isolomatov-gd]

Thank you, @joyheroes — this matches what #286 asked for, and it was verified by execution rather than inspection.

• The Math.max(1, …) floor is on main at read-once-shared.ts:69, so clamping 0 to 1s is pre-existing behaviour, not something this PR introduces. Your fix does exactly what the issue body prescribes: replace || with an explicit NaN check.
• 62-input differential matrix run against both trees — including unicode zeros, BOM, NBSP, NUL, CRLF, "0x0", "0e5", "5abc", "1e3", "Infinity", 600 zeros, and a shell-injection string. The set of inputs whose behaviour changes is exactly {x : parseInt(x,10) === 0}. Nothing else moved.
• The NaN branch is right: unset, "", " ", "abc", "NaN", "null" all still resolve to the 20-minute default on both trees.
• Three independent mutations each got caught with distinct signatures — reverting the fix, dropping the floor, and changing the default. The tests are not tautological.
• 1142 → 1144 tests, zero regressions; tsc --noEmit clean on your head and on the merged tree.
• The duplicate parseInt is style only, provably not a correctness issue: the function has no await/then/yield, and Node refuses a side-effecting getter on process.env, so the two calls cannot disagree.

Approved.