feat(guard): unify placeholder syntax

[sentomk]

Summary

Unify Patternia's guard placeholder surface around _ for a single bound value and PTN_BIND names for multiple values. This is a breaking cleanup: the redundant wildcard alias and positional/macro guard alternatives are removed instead of retained as compatibility aliases.

Changes

• make _ serve as both wildcard fallback and the index-zero guard expression
• move the internal arg_t<N> primitive into a lightweight placeholder header
• allow PTN_BIND declarations at both namespace and block scope
• remove __, _0, public arg<N>, PTN_LET, and PTN_WHERE
• add compile-fail contracts for every removed public spelling
• migrate tests, samples, benchmarks, diagnostics, and current documentation
• add a migration section covering the replacement syntax
• complete the install header manifest so the umbrella header is consumable

Testing

• GCC 12 / C++17 full build, samples, benchmarks, and 179/179 CTest cases passed
• Clang 14 / C++17 build, samples, and 179/179 CTest cases passed
• clang-format 18 dry-run and git diff --check passed
• modular header include-order, standalone wildcard, and PTN_BIND multi-TU probes passed
• installed-package umbrella-header consumer compiled and ran against only the install prefix

[codecov]

Codecov Report

❌ Patch coverage is 91.02564% with 14 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
include/ptn/pattern/type.hpp	67.64%	9 Missing and 2 partials ⚠️
include/ptn/core/common/common_traits.hpp	0.00%	3 Missing ⚠️

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