Refactor protocol generators to use composition

[jbcoe]

Avoid multiple levels of indirection when a protocol_view is constructed from a protocol.

[Copilot]

Pull request overview

Refactors the generated protocol/view glue code to use composition between const/non-const views (and vtables), enabling cleaner reuse and supporting implicit conversion from mutable protocol_view<T> to protocol_view<const T>.

Changes:

• Split protocol concepts into protocol_const_concept_* + protocol_concept_* (const requirements composed into the non-const concept).
• Rework protocol_view generation to compose const/non-const callbacks/vtables and add mutable→const view conversion.
• Add tests covering const view construction from a mutable view (both concrete and protocol-backed cases).

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
scripts/protocol.j2	Refactors generated view callback types and protocol_view to compose const/non-const behavior and add mutable→const conversions.
scripts/protocol_manual_vtable.j2	Refactors manual-vtable generator to use a composed const/non-const vtable layout and updated view construction.
protocol_test.cc	Adds unit tests for constructing a const view from a mutable view.
.github/workflows/cmake.yml	Ensures CI triggers on changes to Jinja2 (.j2) generator templates.

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