docs+feat: README discoverability, rvctool defaults, bdsim model CLI access

[petercorke]

Summary

Note: based on fix/notebook-errata (#45), not main -- retarget to main once that merges, since this branch was cut from its tip and the diff will stay clean once those commits land there.

Triggered by "how does a user find the notebooks/figures after pip install?" -- turned into a broader pass on README clarity, rvctool's prompt defaults, and actually making bdsim models runnable from a bare shell.

README:

• Fix 3 broken links in "Additional book resources" (pointclouds, 3dfigures, examples all pointed at nonexistent top-level folders -- real paths are figures/pointclouds, figures/3d, RVC3/examples)
• Move "Additional book resources" up to right after "Installing the package", instead of being the very last section of a 325-line file
• Add a one-line pointer to that section right after the install instructions, and a top-of-file [!NOTE] banner linking errata.md
• Rewrite "Block diagram models" to document all three ways to run a model (rvctool %run -i, the new rvc3-model command, bdsim_path to locate the install folder)

rvctool: replaced the confusing -n/--normal (dest="book", inverted sense) with a direct --book/--no-book pair. Defaults changed so a plain rvctool gives a nicer interactive experience (RVC3 >>> prompt, Out[N]: labels shown, ANSImatrix on, cwd to examples) rather than the old book-print-fidelity defaults; --book now narrowly means "match the printed page exactly" (plain >>> , no Out[N]:, no ANSI). Also fixes a real bug: book mode used to overwrite --prompt unconditionally even when explicitly passed, silently breaking the tool's own documented RVCTOOL_OPTIONS example.

New RVC3/models/__init__.py, RVC3/bin/__init__.py: both packages were implicit namespace packages (no __init__.py despite being explicitly listed in pyproject.toml's packages=[...]), which is why bdsim_path.py's own Path(models.__file__) crashed (namespace packages have __file__ = None). Restored bdsim_path.py from UNTRACKED/ -- it had existed there, uncommitted, for years; the pyproject.toml [project.scripts] entry pointing at it has never actually worked in any real install until now.

New rvc3-model command: runs a named model from RVC3/models/ directly from a bare shell, no rvctool/Jupyter session needed. Adds the models directory to sys.path first so sibling imports some models use still resolve. (Running it standalone against all models surfaced the missing-imports bug fixed in #46.)

Test plan

• rvctool --test -- 6/6 checks pass
• rvc3-model vloop_test and bdsim_path both work as installed console commands
• Verified all internal README links resolve to real paths