`profiling.sampling` visualisation outputs have edge-case failures

[lkollar]

Bug report

Bug description:

A couple of profiling.sampling visualisation/output paths have edge-case failures:

• Heatmap caller/callee navigation only highlights a target line the first time. Re-clicking a link to the already-selected line leaves the URL hash
  unchanged, so the :target animation is not replayed.

• Differential flamegraph generation can crash for structural elided roots because baseline_self is read before it is initialised.

• Diff flamegraph profiling with subprocesses builds the child profiler command with --diff_flamegraph instead of the valid --diff-flamegraph option.

These are separate symptoms, but all affect generated profiling.sampling visualisations or follow-up analysis workflows.

CPython versions tested on:

CPython main branch

Operating systems tested on:

macOS

Linked PRs

• gh-149430: Fix edge-cases in profiling.sampling outputs #149431