refactor: split asimov into a launcher, a library, and data files

[django23]

The single 1645-line script becomes three things: a launcher, a library, and data files.

bin/asimov              # launcher: version, self-locate, probe, source, dispatch
lib/asimov/*.sh         # 11 modules, largest ~330 lines
data/*.tsv              # sentinels, fixed dirs, skip paths

Installed, the library and data land at <prefix>/libexec/asimov and <prefix>/share/asimov. The launcher resolves its own physical path through symlinks (as Homebrew creates) and probes for both that layout and the repo layout, so ./bin/asimov works straight from a checkout. ASIMOV_LIB and ASIMOV_DATA override the probe.

Data entities

Sentinels, fixed directories and skip paths are tab-separated records that carry their ecosystem or owning tool as a real field, rather than as a trailing bash comment:

node_modules	package.json	javascript	npm, Yarn
DerivedData	*.xcodeproj	swift	Xcode DerivedData

Adding a pattern is now a one-line data edit with no bash syntax involved. tests/data.bats enforces the schema (tabs not spaces, no duplicates, paths relative to the root) and proves a contributor-added sentinel is honoured without touching a script.

Behaviour

Unchanged. All 219 pre-existing tests pass untouched under bash 5 and bash 3.2; only tests/test_helper.bash moved, to point at bin/asimov.

Three things are more than a pure move:

• sentinel_active_pairs() replaces two drifted copies of the config disabled/extra filtering (build_find_vendor_params and discover_new_paths_via_mdfind each had their own).
• A duplicated dead if [[ ! -s "$uncached_file" ]] block in the discovery path is gone.
• doctor gains a Data section reporting loaded counts, and checks the data files before loading them so a broken install is diagnosed rather than aborted on.

Notes for review

• bash 3.2: readonly ARR=(...) inside a function is local under 3.2 and global under 5. Arrays are assigned at file scope; readonly is scalars only.
• Linting is whole-program: make lint runs shellcheck -x --source-path=. bin/asimov, which follows the explicit source lines. That is why the sources are written one per line rather than looped — a loop hides the module bodies and every cross-module variable looks undefined.
• The version stays in bin/asimov, because doctor reads other installs' versions by grepping their launcher rather than executing them.
• Release assets become asimov-X.Y.Z.tar.gz instead of a bare script; scripts/install-remote.sh downloads a tarball.

Verified

• make check — 232 tests, shellcheck clean
• make test-system-bash — same 232 under bash 3.2
• Installed to a throwaway prefix, ran through a symlink, uninstalled clean
• Data files diffed against the old arrays: identical sets

Not in this PR

The homebrew-core formula still does bin.install buildpath/"asimov". That needs updating before the next release tag — see the discussion on the issue.