[memark]

Closes #45.

Most of #45 was already fixed by 93b1454 and 7bd7b35, whose closes #37, #40, #45 only applied the keyword to #37. State on main:

• Every section in 03_intermediate has an exercise now.
• Valgrind note for Apple Silicon is in 02_allocators.md.
• wr check is gone.
• The 2.2 C harness no longer exists (the exercise became a counting allocator in 947067e).
• The bm source tree lists internal.h.
• The signature-only snippets and doSomething are gone.

The one item left was "bm itself is never run", with the fixtures unreferenced. This PR adds a "Take bm for a spin" section at the top of 01_rewrite_a_module.md:

• cd exercises/_bm && make && ./bm
• add two bookmarks, then find and list
• BM_DB=fixtures/sample.bm with find and list
• BM_DB=fixtures/corrupt.bm ./bm list, which shows the storage error path (bm: fixtures/corrupt.bm:4: duplicate, then cannot open ...: corrupt database, exit 1). This relies on fix(_bm): make corrupt.bm reach the storage error path #86, so main is merged into this branch.
• A warning that add, del and tag rewrite the database they point at, so they shouldn't be run on the fixtures. I checked: tag on sample.bm modifies the file, while find and list leave the fixtures byte-identical (bm_close only saves when the database is dirty).

It also gitignores exercises/_bm/bookmarks.bm and bookmarks.bm.tmp, which the walkthrough creates. make clean already removes both.

Every command in the section was run verbatim on a copy of exercises/_bm.