[halidoscope] Add pip packaging via maturin

[alexreinking]

Adds pip packaging for Halidoscope, stacked on #9356.

Packages the compiled Tauri binary (GUI + CLI) as a platform wheel using maturin's bin bindings, so pip install halidoscope puts the halidoscope executable on PATH.

The one non-obvious part: a plain cargo build --release (or maturin build without extra config) compiles fine, but the resulting binary always falls back to loading its UI from Tauri's dev server instead of embedding dist/, silently shipping a broken GUI. Enabling the tauri/custom-protocol Cargo feature (set in pyproject.toml) fixes this -- confirmed empirically by grepping the built binary for the compiled JS bundle's hash before/after.

Verified locally end-to-end: pnpm build -> maturin build --release -> fresh venv -> pip install the wheel -> halidoscope --version/--help both work, and the installed binary now contains the embedded frontend assets.

Halidoscope's version is kept in lockstep with Halide's release version via tbump (matching halide/halide-bin/halide-runtime), but it has no dependency on those packages -- it's a standalone companion tool.

Adds .github/workflows/pip-halidoscope.yml: builds and smoke-tests wheels on all four platforms, decoupled from pip.yml's existing publish job. This workflow is unverified in CI (can't run GitHub Actions locally) and deliberately does not wire into the nightly publish pipeline yet -- that's a follow-up once a real run has been checked.

Also relabels a handful of pre-existing usage examples in tools/halidoscope/README.md from bash to text code fences: they use <placeholder> notation that mdformat-shfmt was silently mis-parsing as shell redirection syntax and corrupting (confirmed directly with shfmt). No content changed, just the fence language.

Breaking changes

None.

Checklist

• Tests added or updated (not required for docs, CI config, or typo fixes)
• Documentation updated (if public API changed)
• Python bindings updated (if public API changed)
• Benchmarks are included here if the change is intended to affect performance.
• Commits include AI attribution where applicable (see Code of Conduct)

[codecov]

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Additional details and impacted files

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[alexreinking]

Follow-up worth tracking separately from this packaging PR: on Linux, halidoscope currently initializes tao's GTK-backed event loop unconditionally inside tauri::Builder::default().run(...) (src-tauri/src/lib.rs), and CLI subcommand dispatch happens inside the .setup() closure, which only runs after that event loop is constructed. That means on a headless Linux host (no X11/Wayland display -- e.g., a CI runner or container), every invocation panics with Failed to initialize GTK backend!, not just the interactive GUI. CI on this PR caught it directly: the manylinux_x86_64 job's smoke test failed running halidoscope --version with exactly that panic (worked around here with xvfb-run, see the workflow).

This undercuts the "non-interactive CLI" pitch for list/stats/dot/snapshot on Linux -- anyone wanting to run those in a headless pipeline needs a virtual display too. The real fix is parsing CLI args (via clap directly, ahead of constructing the tauri::Builder) before ever touching the windowing toolkit, so CLI-only invocations skip GTK entirely. That's a change to src-tauri/src/lib.rs's control flow in #9356, not something packaging can paper over, so I'm flagging it here rather than folding it into this PR.