Four post-Iteration-4 polish items surfaced by manual testing.
1. `--help` / `-h` CLI flag prints a usage banner (options +
app-level commands + DSL grammar reference) and exits. Parse
errors also print the banner to stderr.
2. `help` app-level command notes the same list of supported
commands to the output panel -- a simple stand-in for the
richer H3 help system, kept in sync with what's actually
wired up.
3. The silent rebuild that runs when playground.db is missing
now surfaces a system message in the output panel ("[ok]
rebuild -- N tables, M rows reconstructed; ...") via a new
initial_events plumbing. The user no longer wonders whether
the .db was magically restored or whether anything happened
on launch.
4. Unmodified empty temp projects (kind=Temp, project.yaml has
tables: [] and relationships: []) are now auto-deleted when
the user switches away (load / new / save as) or quits. This
addresses the "launch app, load existing project, quit"
pattern that was leaving an empty temp directory behind
every time. Modified temps (with any user-created tables or
relationships) are never auto-deleted; corrupted projects
are also never auto-deleted (defensive default-to-false on
yaml read/parse errors).
Tests: 338 passing (272 lib + 9 + 5 + 6 + 20 + 9 + 17),
0 failing, 0 skipped. Clippy clean.
claude@clouddev1
b7addd6161