fix(bench): show how to use custom bandwidth profiles

[earlvanze]

Closes #308.

When ft bench bw -o PATH writes outside normal profile discovery, the report now prints the exact shell-safe FREETOKEN_BENCHBW_PATH export needed before ft serve. The --out help text makes the relationship explicit, and the hint is covered by a focused unit test.

Validation:

• python3 -m py_compile for changed modules
• git diff --check
• Focused pytest suite is not runnable in this checkout because pytest/runtime dependencies are not installed.