fix(installer): normalize a trailing slash or dot before the managed-root check

[devswha]

Supersedes #35.

What changed

ensure_managed_root strips a trailing / or /. from INSTALL_ROOT before anything inspects it.

Why

The kernel resolves a symlink when the path ends in / or /., so the ordinary-directory check ran against the link target:

link    -> rejected
link/   -> ACCEPTED   <-- bypass
link/.  -> ACCEPTED   <-- bypass

Severity is low on its own: CHATMUX_INSTALL_ROOT is user-supplied, and the owner check, the 0700 enforcement, and the dev/inode replacement detection all still apply. But rejecting a symlinked root is the stated purpose of that check, so it should hold for every spelling of the path.

Relationship to #35

#35 fixes only the trailing-/ half and rewrites the check from LC_ALL=C stat %F to -d plus ! -L. That rewrite is not needed — #33 already made the check locale-independent — and it splits one atomic stat into a separate type test and metadata read. This change keeps the existing single stat and closes both spellings, so #35 is being closed in favour of it.

link/.. still resolves through the link. That points at the target's parent, a different object rather than a disguised one, and full normalization would need cd -P, which would rewrite the root whenever $HOME itself is a symlink. Left alone deliberately.

Tests

• symlink-trailing-slash and symlink-trailing-dot added to the unsafe-root table in server/install-bootstrap.test.ts
• removing the normalization loop makes that test fail, so the cases are not vacuous
• sh -n install.sh, bash -n install.sh clean
• npm run typecheck, npm run lint, npm run check:identity, npm test (server 907, client 262) all green