[fix] preserve shared stdin buffering and EOF tails (STAR-74)

[vt128]

Successive sys.input() calls previously discarded bytes read ahead by a new temporary reader, and an EOF tail without a newline was returned as an error. Share one synchronized reader across input, stdin, and stdin_lines, preserving a final non-empty line and buffered data after an iterator stops.

Refs STAR-74. Regression tests cover repeated reads, mixed consumers, EOF, CRLF, long lines, read errors, replacement of host stdin, and real CLI pipelines.

Validation: build, race tests (count=2), vet and formatting pass. The existing CLI include capability suite still passes after rebasing on STAR-73.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.47%. Comparing base (65603da) to head (0581aa6).

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #30      +/-   ##
==========================================
+ Coverage   80.54%   82.47%   +1.93%     
==========================================
  Files          23       23              
  Lines        1023     1033      +10     
==========================================
+ Hits          824      852      +28     
+ Misses        151      139      -12     
+ Partials       48       42       -6     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.