feat(navigation): show tab index in titles (display-tab-number)

[cmoron]

Summary

Adds an opt-in [navigation] display-tab-number setting. When enabled, each tab in the rio-rendered strip is prefixed with its 1-based visual position:

1 vim
2 ~/src/rio
3 htop

[navigation]
display-tab-number = true
# Optional separator between the number and the title (default: a single space)
tab-number-separator = " - "   # -> "1 - vim"

Default is false — no change for existing users.

Closes #1537.

Why

• Requested in Feature request: show a number on the tab? #1537 (show a number on the tab, like WezTerm).
• @raphamorim already endorsed this exact API in Show tab numbers in CollapsedTab layout #593: "is possible to add this feature behind a configuration property? … [navigation] display-tab-number = true". That PR was closed only because "this will change a lot on v0.3 as the tab system is being rewritten" — the rewrite has since landed, so this reimplements it on the current Island strip.

Behavior

• Reordering renumbers automatically. The number is the render position (tab_index), i.e. the live slot in ContextManager's contexts vector. Keyboard moves (move_current_to_*) and drag-reorder physically reorder that vector, so a tab dragged from slot 3 to the front becomes 1 and the rest shift up (ghostty-style), live during the drag.
• Configurable separator. tab-number-separator is the string inserted between the number and the title. Default is a single space (1 vim); set " - " for 1 - vim, ": " for 1: vim, or "" for 1vim.
• Applied uniformly, including user-renamed tabs.
• Scope: Tab mode only. Plain has no strip; NativeTab (macOS) uses the OS tab bar, which rio doesn't render.
• Live config reload is honored — the preserved island picks up both settings.

Implementation

• rio-backend: two Navigation fields — display-tab-number: bool and tab-number-separator: String (default " ").
• frontends/rioterm: Island carries both; get_title_for_tab applies the prefix via a small pure format_tab_label(display, position, separator, title) helper, so both the strip and the drag preview share one code path.

Tests

• rio-backend: parses display-tab-number / tab-number-separator and checks defaults.
• rioterm: format_tab_label prefix + separator logic, and a reorder test asserting the moved tab renumbers (["a","b","c"] → move c to front → ["1 - c","2 - a","3 - b"]).

cargo fmt --check, cargo clippy --all-targets --features wgpu, and the touched test suites pass locally.

Docs

Companion docs PR (config reference for both options): raphamorim/rioterm.com#2