TTAS slow path with adaptive backoff

[hayabusa-cloud]

Summary

• Rewrite Lock acquisition into a fast path / slow path split with TTAS (test-and-test-and-set).
• Adaptive backoff in the slow path:
  • n < 4 waiters: Pause (spin, fast wakeup for short critical sections)
  • n >= 4 waiters: Gosched (yield CPU under heavy contention)
• Also fixes a data race on yieldDuration by guarding it with spin.Lock.

Changes:

• lock.go — Split Lock() into inline fast path + lockSlow() TTAS loop with adaptive backoff; document FAA/TTAS design in type and method comments
• yield.go — Protect yieldDuration with spin.Lock for concurrent Yield/SetYieldDuration safety
• go.mod, ci.yml — Bump Go 1.25 → 1.26

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[Copilot]

Pull request overview

This PR implements a TTAS (test-and-test-and-set) lock acquisition strategy with adaptive backoff to improve performance under contention. The fast path uses a single FAA (Fetch-And-Add) operation, while the slow path spins on read-only loads to minimize cache-line invalidations before attempting acquisition. Additionally, the PR fixes a data race on yieldDuration by protecting it with a Lock, and updates the Go version from 1.25 to 1.26.

Changes:

• Rewrites Lock() into fast path (inline FAA) and lockSlow() (TTAS with adaptive backoff)
• Protects yieldDuration with Lock to enable safe concurrent access from Yield() and SetYieldDuration()
• Updates Go version from 1.25 to 1.26 in go.mod and CI workflows

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
lock.go	Implements TTAS acquisition with fast/slow path split and iteration-based adaptive backoff; adds comprehensive documentation on FAA/MESI cache coherence
yield.go	Adds Lock-based protection for yieldDuration to fix data race; documents concurrent safety
pause_test.go	Updates comment to reflect new defaultPauseCycles value of 30
go.mod	Bumps Go version from 1.25 to 1.26
.github/workflows/ci.yml	Updates all CI jobs to use Go 1.26.x

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