fix(skipmap): lost writes when Store races with Delete

[jmasters-git]

Resolves #36

Problem

Two linearizability bugs in skipmap's concurrent operations:

1. Store writes to a dead node. Between checking !marked and calling storeVal, a concurrent Delete/LoadAndDelete can mark and unlink the node. The value is written to a node that is no longer reachable.

2. LoadOrStore reads from a half-linked node. LoadOrStore and LoadOrStoreLazy only checked !marked before returning a loaded value, but Load also requires fullyLinked. A "loser" LoadOrStore can report a key as present while a concurrent Load returns nil for the same key.

Both were found using porcupine linearizability testing.

Fix

• Store: require fullyLinked && !marked before touching the node, then lock and recheck marked before writing.
• LoadOrStore / LoadOrStoreLazy: require fullyLinked && !marked (via MGet) before returning the loaded value, matching what Load already does.
• Regenerated types.go.

LoadAndDelete, Delete, Load, and Range already had the correct checks.

Tests

• TestStoreLoadAndDeleteRace: races Store vs LoadAndDelete on a pre-populated key, asserts the post-condition matches a valid linearization.
• TestLoadOrStoreLoadRace: races 8 LoadOrStore goroutines on the same absent key, asserts Load succeeds immediately after on each goroutine.

Both tests fail reliably on the old code and pass on the fix.

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.27%. Comparing base (964b2cc) to head (d364084).

Files with missing lines	Patch %	Lines
collection/skipmap/gen_func.go	40.00%	5 Missing and 4 partials ⚠️
collection/skipmap/gen_ordereddesc.go	53.33%	4 Missing and 3 partials ⚠️
collection/skipmap/gen_ordered.go	86.66%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #37      +/-   ##
==========================================
- Coverage   96.42%   96.27%   -0.15%     
==========================================
  Files          34       34              
  Lines        3828     3843      +15     
==========================================
+ Hits         3691     3700       +9     
+ Misses         95       94       -1     
- Partials       42       49       +7     

Flag	Coverage Δ
go-1.18.x	95.06% <55.55%> (-0.38%)	⬇️
go-1.19.x	95.38% <55.55%> (-0.12%)	⬇️
go-1.20.x	95.88% <55.55%> (-0.12%)	⬇️
go-1.21.x	95.86% <55.55%> (-0.15%)	⬇️
go-1.22.x	95.78% <55.55%> (-0.20%)	⬇️
go-1.23.x	95.70% <60.00%> (-0.25%)	⬇️
go-1.24.x	95.78% <55.55%> (-0.28%)	⬇️
unittests	96.27% <60.00%> (-0.15%)	⬇️

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