Verify the routing source codes and model the grid's initial state

[baakhoff]

The routing window writes source codes (entity 0x33, selector 0x06) inherited from upstream that have never been verified here by write-and-observe. And since reads stall or alias (docs/PROTOCOL.md), the grid cannot show actual state either — the exact complaint in TheOnlyJoey#11, where the grid starts all gray, gains blue dots on click, and can never be cleared or trusted.

• Verify each destination/source code on the iD24 by ear or loopback, and fix the table where it is wrong.
• Model the power-on defaults so the grid starts truthful instead of gray, and add a way to return to them (pairs with the state persistence in Save and restore mixer state across restarts #5).
• Related unknowns from upstream worth settling with the same test session: whether output 3/4 and the digital outs are fully covered (Figure out output 3/4 and Digital Out's TheOnlyJoey/MixiD#3), and whether the insert/send-return path has any digital control at all (Do send/returns have any digital control? TheOnlyJoey/MixiD#5) — the iD24 on hand has two inserts, so both are testable.