fix(manualmutexunlock): distinguish struct instances sharing a mutex field

[Copilot]

manualmutexunlock tracked mutex state in map[types.Object]*mutexVarState, keyed via pass.TypesInfo.Selections[r].Obj() for field selectors. That returns the field declaration — one *types.Var shared across all instances of the struct — so a.mu and b.mu collide on the same key and clobber each other's state. A deferred unlock on b.mu silently masks a manual-unlock violation on a.mu.

Changes

• New mutexKey composite type {base, field types.Object} replaces bare types.Object as the map key.
  • Direct local (mu) → {base: ObjectOf(mu), field: nil} — unchanged semantics
  • Field selector (a.mu) → {base: ObjectOf(a), field: fieldObj} — instance-distinct
  • Non-ident base (getGuard().mu) → {base: fieldObj, field: nil} — fallback, prior behaviour preserved
• getMutexReceiverObj → getMutexReceiverKey returns (mutexKey, bool) instead of types.Object.
• getLockCallObj/getUnlockCallObj → getLockCallKey/getUnlockCallKey updated accordingly; all call sites in inspectMutexNode updated.
• New testdata cases: BadTwoGuardsManualFirst / BadTwoGuardsManualSecond assert the violation is flagged regardless of statement order; GoodTwoGuardsBothDeferred confirms no false positive when both are deferred.

func BadTwoGuardsManualFirst(a, b *guarded) {
    a.mu.Lock()        // flagged — manual unlock, no defer
    b.mu.Lock()
    defer b.mu.Unlock()
    a.mu.Unlock()      // before fix: b's defer masked this; now correctly reported
}

[Copilot]

Pull request overview

This PR fixes a correctness issue in the manualmutexunlock analyzer where mutex state tracked for struct field selectors (e.g. a.mu) could collide across different struct instances because the key was derived from the shared field declaration object. The update introduces an instance-aware key so violations are reported reliably.

Changes:

• Replace the mutex tracking map key from types.Object to a composite mutexKey{base, field} to distinguish a.mu vs b.mu.
• Update receiver/call extraction helpers to return (mutexKey, bool) and wire through all call sites.
• Add new testdata cases covering two-guard scenarios to prevent regressions.

Show a summary per file

File	Description
pkg/linters/manualmutexunlock/manualmutexunlock.go	Switch tracking to an instance-aware key and update receiver resolution helpers.
pkg/linters/manualmutexunlock/testdata/src/manualmutexunlock/manualmutexunlock.go	Add regression tests ensuring separate struct instances don’t clobber each other’s mutex state.

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• Files reviewed: 2/2 changed files
• Comments generated: 1