feat: detect rust/cargo tool versions, add brew-by-name update fallback

[stanlyzoolo]

Fixes three problems with tools whose binary won't report its own version. Implements the plan in docs/plans/completed/20260724-rust-version-update-detect.md (Variant A: automatic fallbacks, no manual config).

What changed

1. rust had no updater. It ships rustc/cargo, so exec.LookPath("rust") misses and Detect returned ErrUnknownManager — the "no known updater" dead end. On a LookPath miss Detect now checks for a Cellar/<name> or Caskroom/<name> directory and plans brew upgrade <name>. The branch is self-validating (fires only when such a keg exists) and shares version.brewDirVersion's traversal guard.

2. Rust TUIs read as ✕ not found and logged an anomaly on every startup. A ratatui app ignores --version, boots its TUI and panics with no terminal. The capture carried the alt-screen signature plus ratatui-0.30.2 — which versionRe would have mis-parsed as the tool's version once the non-zero exit stopped masking it. InstalledVersion now drops such captures (isTUITakeover, its own copy since version sits below model in the import graph) and falls back to cargo install --list, which knows the version without running the binary at all.

3. installed: collapsed two different states into ✕ not found. InstalledVersion now returns (ver string, present bool) and the card renders them apart: ✓ no version (green ui.OkStyle) for a working install that won't name a version, ✕ not installed (red DangerStyle) for a genuine absence.

Source order is --version/-V → brew directory → cargo list, with the cargo probe gated on the binary existing (an absent tool can only miss, and the probe costs a subprocess).

Verification

Against a real tracker of 23 tools:

tool	before	after
inertia	✕ not found + a log line every startup	0.1.0 via cargo list, no log
rust	[u] → "no known updater"	1.96.1 via brew dir, [u] → brew upgrade rust
keeptui	✕ not found	✓ no version

Every new test was mutation-checked — confirmed to fail when the path it covers is broken:

• remove the cargo fallback → TestInstalledVersionCargoFallback red
• swap brew/cargo order → TestInstalledVersionBrewBeatsCargo red
• drop the binaryExists gate → TestInstalledVersionCargoGatedOnPresence red (3/3 asserts)
• drop info.InstalledPresent = msg.present → TestInstalledMsgCarriesPresence red
• drop present in fetchInstalledCmd → TestFetchInstalledCmd red

Two of those tests were themselves broken on the first pass and only the mutation check caught it: a marker written with touch never resolved (PATH is the fixture dir alone), and an assert on present == false was vacuous since false is the zero value.

preflight green: go build / go vet / go test -race ./... / golangci-lint run (0 issues).

Known limitations, deliberately not addressed

• A clean start still creates a log when keeptui itself is tracked and answers keeptui dev — unparseable, and a legitimate anomaly by the log's own rule. Pre-existing, unrelated to this branch.
• A brew cask pinned to version :latest with no PATH binary still reads ✕ not installed: brewDirVersion can't tell "no keg" from "keg with no parseable version". 0 of 15 casks on the test machine are in that state.
• .. and . pass the formula-name traversal guard. Harmless — Stat only, no writes, and brew rejects the command — and mirrored from version.brewDirVersion, so it should be fixed in both or neither.
• Whether keeptui offers a rust update (the ↑ marker) depends on rust-lang/rust publishing GitHub Releases. The dead end is gone either way; the data source is out of scope.

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