fix: Left/h steps up to the nearest visible ancestor when there's nothing to collapse

[HewstonFox]

Summary

←/h (the collapse action) no-oped whenever the selected row had nothing left to collapse: a file, or a directory that was already collapsed. This left the cursor stuck, with no way to keep climbing the tree via Left alone.

Changes

• src/controller/mod.rs: collapse() now walks up to the nearest visible ancestor directory and collapses that instead of no-op'ing, mirroring how collapsing an expanded directory leaves it selected. Repeated presses climb the tree one level at a time. The walk climbs the real filesystem ancestry rather than stopping after one parent() hop, so it correctly skips past folded intermediate directories under compact_dirs (a folded chain like src/main/java is a single row keyed on its deepest directory — the immediate filesystem parent has no row of its own).
• src/tree.rs: adds TreeModel::select(path), a small helper that repositions the cursor onto an already-visible row without expanding ancestors or relaxing filters (unlike reveal) — exactly what's needed right after collapsing an ancestor.
• tests/controller.rs: two new regression tests — one for the plain already-collapsed-directory case, one specifically for a compact_dirs-folded chain, to lock in that the walk climbs past every folded intermediate row.
• Docs: updated docs/keys.md and docs/usage.md#the-tree to describe the new walk-up behavior, plus a CHANGELOG.md entry under [Unreleased].

Test plan

• cargo test is green
• cargo fmt --all --check, cargo clippy --all-targets -- -D warnings, cargo audit pass
• Docs updated in this PR (CHANGELOG + the relevant docs/ page) for any user-facing change

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