Adopt the STYLE_CPP.md additions from xff

[helly25]

Docs only. Ports the clarifications that had accumulated in xff's STYLE_CPP.md, adapted to mbo's names and to what mbo actually has.

One is a reversal, and it matters

The doc said "a by-value std::string_view is never const". It now says the opposite: a by-value view follows ordinary const-correctness — const when never mutated, non-const exactly when resliced or reassigned. The reason is concrete: misc-const-correctness is enabled and flags a never-mutated view that is not const, so the old rule fought the linter. The corollary (iterate a mutable view and trim in place, rather than const + copy-to-mutate) and a worked example are included, along with the companion rule that a read-only string parameter is std::string_view by value, not const std::string&.

Also adopted

• Trailing comma on every element of a registry-style table, so clang-format expands it uniformly.
• A "value or error" type IS absl::StatusOr<T> — do not hand-roll value + status + ok-flag.
• Matchers named unqualified via using ::testing::Foo;, never ::testing::Foo(...) inline.
• mbo::testing::EqualsText for multi-line text, with a DropIndent raw-string golden (and the trailing-whitespace caveat, since the trim trailing whitespace hook would silently edit a golden).
• ElementsAreArray({...,}) with a trailing comma for long matcher lists.
• SizeIs / IsEmpty on the container, never a matched .size() / .empty().
• bashtest's content matchers (expect_output_contains, expect_matches) with the whole-text anchoring caveat.

Fixed while here

The clang-tidy paragraph was stale: it still claimed "CI does not run it", which #270 changed. It now describes the dedicated clang-tidy CI job, the changed-files-on-a-branch vs whole-tree-on-main scoping, and that it is report-only until the sweep lands.

Deliberately not copied

xff's XFF_ macro prefix, xff/ paths and flag names, its C++23 baseline, its com_helly25_bashtest repo name (mbo uses the default helly25_bashtest), its no-shell-grep-in-bashtests pre-commit hook (mbo has no such hook, so claiming enforcement would be false), and its diff_golden.bzl — the golden-file guidance instead points at mbo's own //mbo/diff:diff.bzl.