feat(backtrace): add showcase workflow

apps/backtrace/EXPECTED.md

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+# Expected Backtrace Demo Markers
+
+Use this file as a quick checklist while reviewing logs. The commands are listed
+in [README.md](README.md).
+
+## Demo 1: Raw Backtrace
+
+Must contain:
+
+```text
+Running backtrace tests...
+BACKTRACE_BEGIN
+BT 0 ip=0x... fp=0x...
+BT 1 ip=0x... fp=0x...
+BACKTRACE_END
+test pass
+```
+
+Must not require:
+
+```text
+=== host backtrace symbolize ===
+```
+
+`--no-symbolize` intentionally disables host symbolization.
+
+## Demo 2: Auto Host Symbolize
+
+Must contain:
+
+```text
+Running backtrace tests...
+
+test pass
+
+
+=== host backtrace symbolize ===
+BACKTRACE_BLOCK
+BT 0 ip=0x... fp=0x... [function name]
+BT 1 ip=0x... fp=0x... [function name]
+ok: backtrace
+```
+
+The exact function names can vary with optimization and toolchain details, but
+the host-side block should not be empty.
+
+## Demo 3: DWARF Auto Symbolize
+
+Must contain:
+
+```text
+emitting raw backtrace report (normal fp chain)...
+
+test pass
+
+=== host backtrace symbolize ===
+BACKTRACE_BLOCK 0 kind=raw
+```
+
+When DWARF/debug info is available, the symbolized block should include the
+synthetic call chain from `test-suit/arceos/rust/backtrace-raw-normal/src/main.rs`:
+
+```text
+c
+b
+a
+```
+
+## Demo 4: StarryOS Memtrack Backtrace
+
+Must contain:
+
+```text
+Memory allocation sample recorded
+Hard memory allocation sample recorded
+BACKTRACE_BEGIN kind=alloc
+BT 0 ip=0x...
+BACKTRACE_END
+STARRY_MEMTRACK_BACKTRACE_OK
+
+=== host backtrace symbolize ===
+BACKTRACE_BLOCK ... kind=alloc
+```
+
+The `sample_hard` allocation path may show extra frames such as:
+
+```text
+starry_memtrack_sample_hard_leaf
+starry_memtrack_sample_hard_mid
+```
+
+Those frames are useful for inspection, but the stable guest-side assertion is
+the deterministic `BACKTRACE_BEGIN kind=alloc` block.
+
+The helper script automatically host-symbolizes the captured app output. When
+symbols are available, the symbolized output may also include:
+
+```text
+BACKTRACE_BLOCK
+starry_memtrack_symbolize_probe
+```