fix(gui): thesis — replace let with const in @score/midi, @score/cli, @score/components

[bwyard]

Summary

Three packages contain let declarations — a thesis violation. Score's core rule: zero let, zero class. All state must be expressed as new values, not mutations.

Locations (from roadmap)

• @score/midi — let violations (critical)
• @score/cli — play.ts has let violations (high)
• @score/components — fm.ts has let violations (high)

How to fix

For each let:

• If it's reassigned → refactor to a pure function that returns the new value, use const at each assignment site
• If it's a loop variable → use reduce, map, or recursion instead
• If it's a WebAudioAPI boundary (audio nodes that must mutate) → it's allowed internally but must be documented with a // AUDIO BOUNDARY — internal mutation permitted comment

Rules

From docs/THESIS_COMPLIANCE.md:

• const everywhere — no let, no var
• Functional loops — map, filter, reduce over for/while
• Mutations at WebAudio boundaries are permitted but must be annotated

Notes

• Run grep -rn "^\s*let " packages/midi/src packages/cli/src packages/components/src to find all violations
• Each package can be a separate commit
• Must not break existing tests

Good first issue — mechanical refactor, clear rules, good way to learn the codebase's functional style.