fix(blocks): preserve tune data in insertMany

[Sion612]

Public blocks.insertMany() discarded OutputBlockData.tunes when composing the inserted blocks. Forward each block's tune data through the existing composeBlock() path so saving preserves it.

Regression tests cover distinct tune values, explicit insertion position, IDs, order, returned BlockAPI holders, unchanged existing blocks, and input without tunes. The existing ExampleTune is reused without changing its behavior.

Relates to #2808. This only addresses bulk insertion; #1681 concerns a separate extension to the single-block blocks.insert() API. This change does not modify that API, insertion defaults, or mutation notifications.

Local validation of this commit:

• Original base plus regressions: 14 passing / 1 expected tune-loss failure.
• Fixed: 15/15 focused Cypress tests in Chrome.
• yarn build:test, yarn lint, yarn build, and git diff --check passed, with existing warnings recorded.

Full Cypress, Firefox, and Edge were not run.