enh(hotspot_analyzer): add --kernel filter for CSV metadata matching

[Arist12]

Problem

hotspot_analyzer.py reads authoritative VGPR/SGPR/LDS/occupancy data from
the *_kernel_trace.csv file written by rocprofv3 --kernel-trace. To
select the correct row it tries to match Kernel_Name against the dispatch
directory basename.

This heuristic works for timestamped output directories
(20240101_120000_pa_decode_kernel) but fails completely for the
ui_output_agent_<N>_dispatch_<id> layout produced by rocprofv3's ATT
decode step. In that layout the directory basename carries only an agent
number and a dispatch counter — no kernel name — so every kernel name
comparison returns false and the metadata lookup silently returns {}.

The result is that the "Register Pressure & Occupancy" section uses ISA
estimates instead of the real CSV values for all ui_output_agent_* traces,
and the warning message gave no hint about how to fix it.

Solution

Add --kernel SUBSTR (optional, default ""):

• When provided, uses a direct substring match on Kernel_Name instead of
  the dir-name heuristic.
• If the *_kernel_trace.csv has a Dispatch_Id column and the
  directory name encodes dispatch_<id>, the row must also match on dispatch
  id. This prevents false matches when a PyTorch reference kernel shares the
  same name prefix as the target kernel and runs in the same profiling session.
• Falls back to kernel-name-only substring matching when the CSV has no
  Dispatch_Id column.

The legacy heuristic (dir basename vs Kernel_Name bidirectional substring) is
unchanged and still used when --kernel is not given, so existing
timestamped-dir workflows are unaffected.

The "not matched" warning now mentions --kernel so users can discover the
fix without reading source.

Before / after

# Before — metadata not loaded for ui_output_agent_* dirs
python hotspot_analyzer.py ui_output_agent_15249_dispatch_223 --topk 4 --mode src
# (kernel_trace CSV not matched — accum/LDS/SGPR estimated from ISA only)

# After — CSV metadata loaded correctly
python hotspot_analyzer.py ui_output_agent_15249_dispatch_223 \
    --topk 4 --mode src \
    --kernel pa_mqa_logits_fp4_kernel_0
# Prints real VGPR/SGPR/LDS/occupancy from out_kernel_trace.csv

Testing

Five unit tests covering:

1. Legacy timestamp heuristic still works (no regression).
2. ui_output_agent_* dir without --kernel returns {} (expected).
3. --kernel + Dispatch_Id column selects the correct CSV row.
4. --kernel without Dispatch_Id column falls back to name-only match.
5. argparse wires --kernel through to read_kernel_metadata.

All five pass.