[fix] hold the Machine lock for the whole REPL session

[vt128]

Problem

REPL() drove prepareThread and repl.REPLOptions against m.thread and m.predeclared with no lock, while every other execution path (Run, RunFile, RunWithTimeout, RunWithContext, …) holds m.mu for the whole run and Reset holds it to swap the thread (STAR-69). A REPL session concurrent with a Run or Reset on the same Machine was therefore a data race on m.thread.

Fix

REPL() now takes m.mu.Lock() / defer Unlock() for its whole session, matching runInternal. A REPL owns the Machine exclusively for its duration, so holding the lock until it exits is the right scope. String() already uses TryRLock and returns the running snapshot rather than blocking, so it stays responsive during a REPL. prepareThread and getFileOptions do not re-acquire m.mu, so there is no self-deadlock.

Test

TestMachine_REPLIsRaceFree drives the REPL non-interactively (stdin pointed at os.DevNull, so repl.REPLOptions returns on EOF) and runs it concurrently with Run and Reset. It reports a data race under -race before this change and is clean after.

Verification

gofmt / go vet clean; go test, go test -race -count=2, full ./..., and docker golang:1.19 go test -race all green.

Part of the STAR-69 follow-up (residual flagged during that fix's review).

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

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🟢 Metrics 7 complexity · 2 duplication

Metric	Results
Complexity	7
Duplication	2

View in Codacy

🟢 Coverage 100.00% diff coverage · +0.03% coverage variation

Metric	Results
Coverage variation	✅ +0.03% coverage variation (-1.00%)
Diff coverage	✅ 100.00% diff coverage

View coverage diff in Codacy

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (d97f2a1)	7874	7458	94.72%
Head commit (468b610)	7915 (+41)	7499 (+41)	94.74% (+0.03%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#174)	10	10	100.00%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.54%. Comparing base (d97f2a1) to head (468b610).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #174      +/-   ##
==========================================
+ Coverage   93.52%   93.54%   +0.01%     
==========================================
  Files          50       50              
  Lines        6298     6316      +18     
==========================================
+ Hits         5890     5908      +18     
  Misses        259      259              
  Partials      149      149              

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