Enable -Weverything warnings and add optional active_message field

[dallison]

Summary

• Compile Phaser's own code under clang's -Weverything, with a curated set of -Wno-* opt-outs in phaser/copts.bzl for non-actionable diagnostics (C++ back-compat, padding, vtable emission, intentional offset/size conversions, etc.). Third-party headers are treated as system headers via --features=external_include_paths.
• Fix all resulting in-repo warnings: a bug-prone shadowed UnionInfo::binary_size, potential-UB shift-negative-value / undefined-reinterpret-cast, deprecated implicit copy ops on Message/Field, plus extra-semi, unused params/functions, ignored [[nodiscard]], missing initializers, and ~20 nested-scope shadow renames in tests.
• Add an optional public std::any active_message field to generated message types, enabled via phaser_library(enable_active_message = True) or the active_message=true plugin command-line option. Off by default (no change to existing output).

Test plan

• bazel test //... (6 targets) passes
• Full in-repo -Weverything recompile is warning-clean
• New //phaser:active_message_test verifies the field defaults empty, holds a payload, and resets
• Generated header confirmed to emit #include <any> and std::any active_message; only when enabled