fix(core): stop spinning on terminal read errors

[liamg]

Summary

• surface non-EOF terminal read failures from the key reader instead of treating them as empty input
• return terminal read errors from Readline() so callers can exit cleanly when the tty disappears
• add regression coverage for EOF and non-EOF key read failures

Problem

When stdin starts returning a non-EOF read error, the Unix key-reading path currently ignores it and returns an empty key buffer. WaitAvailableKeys() then loops forever, allocating a fresh buffer each time and burning CPU in the input loop.

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 32.84%. Comparing base (0637db4) to head (0c6af78).
⚠️ Report is 16 commits behind head on master.

Files with missing lines	Patch %	Lines
emacs.go	0.00%	2 Missing ⚠️
readline.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #97      +/-   ##
==========================================
- Coverage   33.03%   32.84%   -0.19%     
==========================================
  Files          57       58       +1     
  Lines        9109     9224     +115     
==========================================
+ Hits         3009     3030      +21     
- Misses       6062     6154      +92     
- Partials       38       40       +2     

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[maxlandon]

Awesome !!!!!