Add parse-only REPL input completeness check

[fingolfin]

Adds CHECK_ALL_COMMANDS(instream): parse input without executing or printing anything, and classify it as "complete", "incomplete" (truncated prefix of valid input) or "error". Diagnostics are returned as records with message and caret positions instead of being printed.

This is the primitive alternative frontends need for multi-line editing: a Julia REPL "GAP mode" via GAP.jl, or a Jupyter kernel's is_complete_request, can ask on every Enter whether the buffer is ready to evaluate, instead of submitting half-typed input.

Implementation: the reader runs with the interpreter in ignoring mode (cf. the experiment in #4130, but no asserts are relaxed; IntrEnd is skipped in check mode, so nothing runs, not even quit; or ?help). The scanner records whether the first syntax error was caused by the input ending; only then is input "incomplete", so a typo inside an open construct still reports "error" and cannot trap the user in an editor that never submits.

Planned follow-ups: a structured chunk evaluator (per-statement results, captured output and error text, no break loop) and libgap wrappers GAP_CheckInput / GAP_EvalStringEx.

Written with Claude Code (Claude Fable 5).

[codecov]

Codecov Report

❌ Patch coverage is 82.75862% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.98%. Comparing base (989cc35) to head (2331e26).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/scanner.c	76.31%	5 Missing and 4 partials ⚠️
src/streams.c	79.31%	4 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6543      +/-   ##
==========================================
- Coverage   79.00%   78.98%   -0.03%     
==========================================
  Files         684      684              
  Lines      294205   294285      +80     
  Branches     8647     8664      +17     
==========================================
- Hits       232444   232435       -9     
- Misses      59955    60036      +81     
- Partials     1806     1814       +8     

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[fingolfin]

Maybe an integer with specific defined values (say 0, 1, 2), plus some BindGlobal("COMMAND_STATUS_ERROR", ...)etc for the values would be better: checking strings is error prone (typos...)