Add register repacking

[mkeeter]

This PR sorts bytecode registers by frequency of use. The goal is to optimize GPU evaluators, which use a select chain to dynamically pick a register from an array.

[Copilot]

Pull request overview

This PR adds register-frequency repacking for VM bytecode generation, aiming to place the most-used registers at lower indices for GPU evaluator efficiency.

Changes:

• Exposes immutable access to VmData’s inner RegTape.
• Adds register visitation and repacking helpers to RegOp/RegTape.
• Applies register repacking during bytecode serialization.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
fidget-core/src/vm/data.rs	Adds VmData::asm() accessor for the inner register tape.
fidget-core/src/compiler/reg_tape.rs	Adds frequency-based register repacking and map generation.
fidget-core/src/compiler/op.rs	Adds register output/visitor helpers for RegOp.
fidget-bytecode/src/lib.rs	Applies repacking when emitting bytecode.
CHANGELOG.md	Documents the new public APIs.

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