Add clang-tidy static analysis

[NiceAndPeter]

Adds clang-tidy as a static-analysis CI gate — the layer above the sanitizers /
coverage / perf gates from the previous pass. It catches at parse/AST time what
runtime checks can't (bugprone integer/sign patterns, accidental copies, missing
moves, portability traps).

What's in it

.clang-tidy — a curated, correctness-only config. This is a bug-finding
gate, not a restyle. A broad set produced ~355 findings on a single TU, almost
all style or by-design (const-correctness, literal-suffix casing, public members
for NTTP/memcpy, intentional numeric narrowing). The config keeps
bugprone-* / performance-* / portability-* and disables only the specific
checks that are false positives or fight bound's deliberate design — each with a
one-line rationale in the file:

Disabled check	Why
misc-redundant-expression	fires on <L> op <R> template pairs it can't distinguish pre-instantiation (and fold-expr identity seeds) — net FP
bugprone-narrowing-conversions	intentional, guarded numeric narrowing (MSVC C4244/C4305)
bugprone-signed-char-misuse	i8 is a numeric raw-storage type; sign-extension is intended
bugprone-exception-escape	noexcept helpers route errors through the replaceable raise() handler (traps under -fno-exceptions) by design
bugprone-branch-clone / performance-enum-size / bugprone-easily-swappable-parameters	template FP / deliberate int enums / pervasive numeric pairs

The only two genuine, intentional sites left are // NOLINT'd in test code
only (a macro-expansion inc-dec FP in test_compound_assign, deliberate
catch-alls in the fuzz harness) — no suppressions in library headers.

CI job (linux clang-tidy): configures with clang-18 + compile_commands and
runs run-clang-tidy over tests + examples, header-scoped to include/bound/.
WarningsAsErrors: '*' fails the run on any finding.

Verification

• Full local run over tests/*.cpp + examples/*.cpp: 0 findings.
• Gate has teeth: a planted issue (performance-for-range-copy) makes it fail.
• No library headers changed (config + two test files only), so the single-header
  amalgamation is unaffected.

Follow-up

After this is green and merged, I'll add linux clang-tidy to the required
status checks on main (currently 18 → 19).

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