docs: README comparison matrices + execution-tested cookbook examples

[helly25]

What

Expand README.md, and make the user-facing examples provably runnable.

README.md

• Add a Tool Feature Comparison Matrix (xff vs find / fd / grep / ripgrep / shasum) and a Flavor & Behavioral Shift Matrix; all tables vertically aligned via tools/align_markdown_tables.py.
• Fix an invalid example (-print "{path} {sha256}" → xff -type f -hash=sha256), a broken table row, and mangled \t / \n escapes.
• Mark find's "Custom Layout Templating" as GNU — -printf is a GNU extension; POSIX and BSD/macOS find have no format primary — with an explanatory note.
• Footnote strict --config=find: it stays xff's own engine, keeping RE2 as the -regex / -iregex default (verified: regex is not a per-style default in config.cc), one linear-time engine identical on Linux and macOS (where GNU find defaults to Emacs and BSD to BRE); -regextype exposes xff's grammar set, never GNU's dialect names.
• Drop the em-dash in the title (repo no-em-dashes rule).

xff/examples (new package)

• cookbook_test.sh runs every --help=cookbook recipe (SOT: cli/help.cc RenderCookbook) against a fixture tree rather than only rendering it — this is what would have caught the invalid example above — plus the README "missing final newline" linter. The two git blame recipes exercise real git and degrade to a logged skip where git is absent.
• A guard case (cookbook_recipes_are_all_tested) fails if a recipe is added or reworded without a matching execution case, so examples ship tested, not just shown.

cli/help.cc, AGENTS.md

• Document that cookbook recipes are execution-tested by //xff/examples:cookbook_test, and add "a new/changed cookbook recipe carries a test" to the self-documenting-features rules.

Testing

• bazel test //xff/examples:cookbook_test //xff/cli:help_topic_test → pass (10/10 in the new suite, both git recipes ran).
• pre-commit run on every changed file → green: clang-format, shfmt, shellcheck, no-shell-grep-in-bashtests, no-em-dashes, align-markdown-tables, module-versions, whitespace/EOF.
• //xff/cli:xff_markdown_test is host-incompatible locally (runs in CI); the help.cc change is comment-only and cannot alter --markdown output.

Not in this PR

The mbo 0.13.0 graduation (MODULE.bazel: drop the temporary git_override for a BCR bazel_dep) is intentionally left out and will land as its own change.