fix: TrackNodeSend test calls non-existent updateSendPrePost method

[ChuxiJ]

Problem

The TrackNodeSend.test.ts unit test is failing in CI on all PRs because it calls trackNode.updateSendPrePost(), a method that does not exist on TrackNode.

Error:

TypeError: trackNode.updateSendPrePost is not a function

Root Cause

When aux sends were implemented in PR #1039, the test was written against an initially planned API that used:

• updateSendPrePost(id, 'pre' | 'post') — separate method for switching tap point
• connectSend(id, dest, amount, 'pre' | 'post') — string literals for pre/post

But the actual implementation uses:

• updateSendAmount(id, amount, preFader: boolean) — combines amount + pre/post switching
• connectSend(id, dest, amount, preFader: boolean) — boolean parameter

The test was never updated to match the final API.

Impact

• 1 test failing across all PRs (blocks CI unit-test job)
• 3475 other tests pass fine

Acceptance Criteria

• updateSendPrePost test updated to use updateSendAmount with correct boolean parameter
• All connectSend calls in test use boolean instead of string 'pre'/'post'
• All 3476+ unit tests pass
• No changes to production code (test-only fix)