feat(rust): extract reusable terminal-UI primitives into mergify-tui

[jd]

Reviewer feedback: the colored-tree rendering helpers in
crates/mergify-queue/src/status.rs (Theme + relative-time
formatter + box-drawing characters) are general-purpose and will
be needed by every command with structured human-readable output
(queue show, freeze list, future ports). Centralize them
in a new mergify-tui crate so each port consumes the same
primitives instead of forking its own copy.

The new crate has three modules:

• theme: Theme struct with TTY/NO_COLOR-aware
  enable/disable. Pre-built named styles (bold, dim,
  cyan/green/red/yellow/magenta, warn)
  plus a fg(AnsiColor) helper for domain-specific palettes.
  Theme::detect is the production constructor;
  Theme::new(enabled) is for tests that need to exercise the
  styled or plain branch deterministically.

• time: relative_time(iso, now, future) — coarse
  Ns/Nm/Nh/Nd formatter mirroring the Python
  CLI's _relative_time. Empty string on parse failure so a
  malformed timestamp doesn't abort the surrounding render.

• tree: Unicode box-drawing constants (BRANCH,
  LAST_BRANCH, CONTINUATION, LAST_CONTINUATION) plus
  branch_chars(is_last) to pick both prefixes for a row in
  one call.

mergify-queue::status is refactored to consume from
mergify-tui: the local Theme struct and relative_time
function are deleted, the inline ├── /└── /│ /
literals are replaced with tree::branch_chars calls,
and the queue-specific batch_status_style helper now uses
Theme::fg for its per-status color mapping.

Tests: 11 new in mergify-tui (theme on/off, relative-time
units + parse failure + future prefix, tree-character pairing).
The 6 relative_time_* tests in mergify-queue are removed
— their coverage moves with the function. Workspace count: queue
24 (was 30), tui 11.

Co-Authored-By: Claude Opus 4.7 (1M context) noreply@anthropic.com

Depends-On: #1366

[jd]

This pull request is part of a Mergify stack:

#	Pull Request	Link
1	feat(rust): port queue pause and unpause to native Rust	#1352
2	feat(rust): port ci git-refs and ci queue-info to native Rust	#1353
3	feat(rust): port queue status to native Rust	#1359
4	test: derive native queue commands from the binary, not a hardcoded list	#1366
5	feat(rust): extract reusable terminal-UI primitives into mergify-tui	#1379	👈
6	fix(cli): surface clap's --help instead of erroring on native subcommands	#1380

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• depends-on=Mergifyio/mergify-cli#1352 [⛓️ feat(rust): port queue pause and unpause to native Rust #1352 [stack]]
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• depends-on=Mergifyio/mergify-cli#1359 [⛓️ feat(rust): port queue status to native Rust #1359 [stack]]
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• depends-on=Mergifyio/mergify-cli#1352 [⛓️ feat(rust): port queue pause and unpause to native Rust #1352 [stack]]
• depends-on=Mergifyio/mergify-cli#1353 [⛓️ feat(rust): port ci git-refs and ci queue-info to native Rust #1353 [stack]]
• depends-on=Mergifyio/mergify-cli#1359 [⛓️ feat(rust): port queue status to native Rust #1359 [stack]]
• depends-on=Mergifyio/mergify-cli#1366 [⛓️ test: derive native queue commands from the binary, not a hardcoded list #1366 [stack]]

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