TASK-003 review cleanup: fix 1 major + 16 minor items

Major (item 1): replace redundant `catches_as_feature_unavailable_directly`
test (which only proved pointer upcast already guaranteed by static_assert)
with `catches_as_std_exception`, a distinct test verifying the full
std::exception -> std::runtime_error -> feature_unavailable chain
and that what() is not sliced.

Other fixes:
- src/httpserver.hpp: bump C++ guard from 201703L (C++17) to 202002L
  (C++20) per DR-001 (item 2)
- feature_unavailable.hpp: replace magic literal 32 with
  `k_fixed_overhead` constexpr computed from the actual string literal
  at compile time, keeping reserve() in sync with future wording
  changes (items 3, 15); inline compose_message helper into an
  immediately-invoked lambda in the constructor initializer, removing
  private surface (item 9); add @param lifetime doc comments for the
  string_view arguments (item 16)
- feature_unavailable_test.cpp: add `LT_CHECK(!msg.empty())` guard
  before find-checks in all tests (item 6); add
  `empty_args_produce_non_empty_message` test for ("", "") edge case
  (items 7, 19); add comments to empty set_up/tear_down bodies (items
  4, 5, 11, 12)

Deferred: item 14 (_index.md status, handled by merge step),
item 17 (pre-existing HAVE_* guards, future task per PRD-FLG-REQ-001),
item 10 (reviewer says no change needed).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: etr