feat(console): render a unified diff for multiline assert_equals/assert_same failures

[Chemaclass]

Why

assert_equals/assert_same failures print the full expected and actual values on single quoted lines (bashunit::console_results::print_failed_test, src/console_results.sh:305). For multiline strings (command output, file contents) this is unreadable — you get two blobs and hunt for the difference by eye.

Snapshot failures already solve this: print_failed_snapshot_test (src/console_results.sh:332) renders a git diff of expected vs actual when git is available (bashunit::dependencies::has_git), falling back to plain output otherwise. Reuse that machinery.

Proposal

When both of these hold in print_failed_test:

• expected or actual contains a newline, and
• bashunit::dependencies::has_git (same gate as snapshots),

render a unified diff (same temp-file + git diff --no-index approach as the snapshot path — extract the shared logic into one helper instead of duplicating) after the existing Expected/but got header, possibly truncating the inline quoted values to their first line + … when the diff is shown.

Single-line values keep the current output byte-identical — a large body of acceptance/snapshot tests depends on it, and so do users parsing output.

Design notes

• Extract the diff-rendering block from print_failed_snapshot_test into e.g. bashunit::console_results::render_diff expected_text actual_text and use it from both call sites (one commit for the extraction, no behavior change; next commit adds the multiline hook).
• Respect --no-color / BASHUNIT_NO_COLOR (git diff must be called with color flags consistent with src/colors.sh state).
• Machine formats (JUnit/TAP/JSON reports) must keep receiving the raw expected/actual values, not the diff — only the console renderer changes.
• Add an opt-out env (BASHUNIT_NO_DIFF=true) for users who scrape console output.

Tests first (TDD)

Acceptance test: multiline assert_equals failure shows a diff hunk; with BASHUNIT_NO_DIFF=true or without git shows current format; single-line failure output unchanged (snapshot test). Unit test the extracted render_diff helper.

Constraints

• Bash 3.0+; git remains optional (graceful fallback).
• CHANGELOG.md entry (user-visible).

Acceptance criteria

• ./bashunit tests/ and ./bashunit --parallel tests/ pass.
• make sa, make lint pass; shfmt -w . produces no diff.
• Existing failure-output snapshot tests untouched or deliberately updated only for the multiline case.

Line references valid at commit 1ae3e82.