feat(guard): anchor PTN_BIND names to members

[sentomk]

Summary

Upgrade PTN_BIND placeholders from positional (arg_t<N>) to member-anchored (member_t<&Type::name>). Guards attached to has<...> rewrite each name to the position of its member in the pattern's member list at compile time, so names follow members — the order of member pointers in has<...> no longer matters. This also activates the previously documentary Type argument and resolves the deferred Type-validation roadmap item without static reflection.

Changes

• PTN_BIND(Type, ...) now expands each name to constexpr member_t<&Type::name>; misspelled member names fail right at the macro line.
• New compile-time machinery in guard.hpp: member_position (skips _ign slots) + resolve_expr/resolve_pred expression-tree rewriting; evaluation machinery untouched.
• Resolution hooks: has_pattern::operator[] (non-binding path) and a new guard_resolver hook in binding_base.hpp, specialized for structural_bind_pattern (binding path).
• _ keeps its positional meaning and mixes freely with member names.
• Clear static_asserts when a name is missing from has<...> or used on a non-structural pattern.
• Docs: docs/api.md PTN_BIND section rewritten (member semantics + PTN_ON capture caveat); roadmap WIP entry added and the deferred validation item marked resolved.

Testing

• tests_named_placeholder.cpp rewritten for member semantics: type checks assert member_t<&T::m>; new cases cover order independence (has<&P::y, &P::x>), _ign slot skipping, mixed _ + member names, and the non-binding has<>[guard] path. Block-scope binds use ptn::on because PTN_ON's captureless caching lambda cannot capture block-scope names.
• tests_destructure.cpp migrated from alias names to member names.
• New compile-fail cases, each verified to fail with the intended diagnostic: member_placeholder_not_in_pattern, member_placeholder_non_structural, ptn_bind_unknown_member.
• Full suite: 191/191 passing locally (GCC 13 + Clang 20, C++17).
• clang-format clean on all touched files.

Notes

• Breaking for alias-style PTN_BIND usage (placeholder names that were not real members): such declarations now fail at the macro line. Member-named usage is source-compatible with identical semantics when written in matching order.

[codecov]

Codecov Report

❌ Patch coverage is 83.72093% with 7 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
include/ptn/pattern/modifiers/guard.hpp	70.83%	7 Missing ⚠️

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