Fix macOS editor launch (os error 2) + add open_at line/column targets

[sksizer]

Addresses the two path-opener findings from the Annado integration review (the other two findings in that PR were app-side UI polish and don't concern this crate).

Fix 1 — macOS editors fail to launch with os error 2

On macOS the GUI editors are detected by their .app bundle but were launched by their CLI shim (code, subl, cursor, zed). That shim is frequently not symlinked onto PATH (Sublime's subl never is by default), and a GUI-launched host (Tauri, etc.) inherits a stripped PATH — so open/open_with could throw NotFound for an app that detection reported as available.

Now, on macOS, a plain open of these editors goes through open -a "<App Name>" (LaunchServices, PATH-independent). Terminals already did this; cmux genuinely needs its CLI and is unchanged.

Fix 2 — jump to a line (was dropped in the rewrite)

New open_at(path, app_id, &Target) navigates to a location inside a file. Target { line, column } is a small carrier for sub-application markers, generalized so future markers (an anchor, a page, …) become new fields rather than a new open_* per coordinate.

• Honored by the editors that can jump inside a file: VS Code, Cursor (--goto file:line:col) and Sublime, Zed (file:line:col suffix). Modeled as a first-class internal Editor type.
• On macOS the editor CLI is resolved from inside the app bundle first, then PATH — so the jump works even when the shim is missing from PATH (Fix 1's whole problem). If neither resolves, the file still opens via open -a, just without the jump.
• Every other opener (terminal, file manager, Obsidian) ignores the target and opens the path normally — open_at is always safe to call.
• PathOpener.accepts_target exposes which detected openers qualify, so callers stop hardcoding the goto-capable list.
• preview_command_at mirrors preview_command.

Verified on macOS (previews of the actual argv):

vscode open              -> open ["-a", "Visual Studio Code", "/tmp/proj"]
vscode @line42           -> /Applications/Visual Studio Code.app/.../bin/code ["--goto", ".../main.rs:42"]
zed    @5:3              -> /Applications/Zed.app/Contents/MacOS/cli [".../main.rs:5:3"]
sublime @10 (not installed) -> open ["-a", "Sublime Text", ".../main.rs"]   # graceful degrade
finder @line42           -> open [".../main.rs"]                            # target ignored

Compatibility

• PathOpener gained accepts_target: bool — hand-built literals must fill it in.
• macOS open/open_with/preview_command now spawn open -a for the GUI editors; the effective argv changed (more robust).
• open_at, preview_command_at, and Target are additive.

Checks

just full-check (fmt + clippy) clean; cargo test green (28 unit + 10 doc) on default and --features specta.