Closes https://github.com/zed-industries/zed/issues/60242

# Objective

When reviewing a branch diff — or project search results — the outline
panel lists every affected excerpt and symbol beneath each file. On a
diff touching more than a handful of files, this consumes so much
vertical space that you cannot see the list of changed files at all,
which is usually the thing you want the panel for.

Existing settings don't cover this:

- `expand_outlines_with_depth: 0` still renders excerpt rows and
top-level symbol rows.
- Collapsing a file row in the panel is coupled to folding that buffer
in the editor, so it can't be used purely to tidy the panel.

## Solution

A display-only mode that reduces multi-buffer views to just the
directory/file tree.

- New `outline_panel.multi_buffer_hide_symbols` setting (default
`false`). When enabled, multi-buffer views render only directory, file
and folded-directory rows — excerpt, symbol and search-match rows are
hidden.
- New `outline panel: toggle symbols` action, plus a button in the panel
footer, which toggles an in-memory per-panel override of the setting
(the same pattern as `editor: toggle inlay hints`). Changing the setting
clears the override, so the setting wins.
- Single-file views are exempt, and the footer button is not shown there
— the panel contains only symbols in that case, so hiding them would
leave it empty.
- The filter is display-only: `collapsed_entries` and editor buffer
folding are never touched.
- Outline fetching is skipped while the mode is active, so large diffs
don't pay for outlines that will not be rendered. Turning the mode off
re-triggers fetching.

This adds one new icon, `file_code_off`, used by the footer button when
symbols are hidden. cc @zed-industries/design for review per the icon
guidelines — it is `file_code` with the slash and outline gaps from
`file_ignored`, keeping the parts of the chevrons that fall on the near
side of the slash.

## Testing

Three new tests in `crates/outline_panel/src/outline_panel.rs`:

- `test_hide_symbols_in_multibuffer` — outline rows are present by
default; only `Fs`/`FoldedDirs` entries remain once the mode is enabled
via the setting (which also exercises the settings observer) and via the
action; outline rows return when it is toggled off, confirming outlines
are re-fetched; asserts `collapsed_entries` is unchanged at every step,
covering the display-only guarantee.
- `test_hide_symbols_exempts_singleton` — a single-file view still
renders its outline with the setting enabled.
- `test_hide_symbols_hides_search_matches` — project search match rows
are hidden while files and directories still render, and return when
toggled off.

`cargo test -p outline_panel` (15 passing), `cargo test -p icons`,
`cargo test -p settings_ui -p settings_content`, and `./script/clippy`
on the touched crates are all green.

Tested manually on macOS against a branch diff, project search results,
and single-file buffers, checking both the setting and the action (from
the panel footer button and from the command palette with focus outside
the panel), and that expanding/collapsing directories and editor folding
still behave as before.

## Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content adheres to Zed's UI standards
([UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
and
[icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md)
guidelines)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

## Showcase

<!-- Screenshots go here: branch-diff before/after, footer button icon
states, search before/after, single-file exemption -->


|  | Symbols shown (default) | Symbols hidden |
| --- | --- | --- |
| **Footer toggle** | <img
src="https://github.com/user-attachments/assets/393f128d-1a7c-43a8-97b8-8176a07b45c7"
width="260"> | <img
src="https://github.com/user-attachments/assets/87c60dd4-d5b7-48f8-9255-226c4a94a586"
width="260"> |
| **Branch diff** | <img
src="https://github.com/user-attachments/assets/79357325-7d7d-40f7-8067-4ae59a824f87"
width="420"> | <img
src="https://github.com/user-attachments/assets/8a3bbc7e-90d4-4638-9f1d-1ba3091eccfd"
width="420"> |
| **Project search** | <img
src="https://github.com/user-attachments/assets/76e3cf18-d624-42f8-8801-f496bf54a552"
width="420"> | <img
src="https://github.com/user-attachments/assets/2a34e899-b9f4-465e-b571-75d3d54d00eb"
width="420"> |

Single-file views are unaffected — symbols still render, and the toggle
is not offered:

<img
src="https://github.com/user-attachments/assets/76c47884-96db-4071-bcc1-931c4283e36f"
width="640">

---

Release Notes:

- Fixed too many outlines shown per excerpt in the outline panel. Added
an `outline_panel.multi_buffer_hide_symbols` setting and an `outline
panel: toggle symbols` action to show only files and directories in the
outline panel for multi-buffer views.

---------

Co-authored-by: Kirill Bulatov <kirill@zed.dev>