perf python: Port flamegraph to perf module

Port flamegraph.py to a standalone script in tools/perf/python/ that
uses the perf module directly, avoiding intermediate dictionary
allocations for event fields.

Improvements compared to the legacy script:
- Add Subresource Integrity (integrity="sha256-...") and
  crossorigin="anonymous" attributes to external CDN stylesheet and
  script tags in MINIMAL_HTML.
- Upgrade CDN HTML template hash verification from weak MD5
  (hashlib.md5) to cryptographic SHA-256 (hashlib.sha256).
- Escape '<', '>', and '&' ('\u003c', '\u003e', '\u0026') in embedded
  JSON payloads (stacks_json and options_json) to prevent HTML script
  injection / XSS when rendering untrusted symbol or command names.
- Skip invoking 'perf report --header-only' when the input is stdin
  ('-'), a FIFO pipe, or a character device (S_ISFIFO / S_ISCHR) so
  non-seekable streams do not hang or fail.

Add a shell test (test_flamegraph_python.sh) to verify the standalone
script.

Assisted-by: Antigravity:gemini-3.1-pro
Signed-off-by: Ian Rogers <irogers@google.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

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