validate trace packet header fields against payload size

[soma0212]

the trace tools read binary packets whose header carries a size field along with dimensions and a halide_type_t, but both readers only bound payload_size against the fixed 4096-byte payload buffer and never check the header agrees with the bytes actually read. PacketAndPayload::read in HalideTraceViz and Packet::read_from_filedesc in HalideTraceUtils then hand the packet straight to coordinates(), value(), func() and trace_tag(), which derive their pointers from dimensions and type.lanes, so a crafted trace whose size covers only the header while dimensions is in the thousands makes coordinates()[i] and the func/trace_tag string walks run far past the payload. reject a packet up front when dimensions is negative, when the coordinate and value bytes exceed payload_size, or when the two trailing nul terminators fall outside it, which is exactly the layout the runtime tracer always writes.

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]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (main@0765408). Learn more about missing BASE report.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #9220   +/-   ##
=======================================
  Coverage        ?   69.77%           
=======================================
  Files           ?      255           
  Lines           ?    78580           
  Branches        ?    18790           
=======================================
  Hits            ?    54833           
  Misses          ?    18176           
  Partials        ?     5571           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[alexreinking]

I'm going to hold off on this one while some other in-flight work on the tracing system gets resolved. There's a good chance we delete these files in a month anyway and I don't want merge conflicts for my intern.

[soma0212]

Makes sense, no rush. If the rework keeps these packet readers in some form the header-vs-payload check is small enough to port over, and if the files go away entirely, feel free to close this.