Phase 2: Freestanding / embedded profile (no exceptions/RTTI option)

[tap]

Part of Phase 2 — Reach (ROADMAP.md).

A build profile that lets the parse/serialize core compile and run with C++ exceptions and RTTI disabled and the hosted-only facilities dropped — the shape an embedded target needs (see the Pico 2W demand signal, sibling issue).

Groundwork — landed (branch claude/phase-2-prep-h8q9mz):

• osc/OscConfig.h build seam: auto-detect OSCTAP_HAS_EXCEPTIONS; route every core throw through OSCTAP_THROW (plain throw when exceptions are on; non-returning, user-overridable OSCTAP_FATAL_HANDLER, default std::abort(), when off).
• OSCTAP_FREESTANDING drops <iostream>, the std::vector-backed OwnedMessage, and the std::string operator<<.
• OSCTAP_FREESTANDING CMake option builds tests/OscFreestandingTest.cpp with -fno-exceptions -fno-rtti (flags PRIVATE to that target); freestanding CI job (GCC + Clang, C++17/20) keeps it green.
• Hosted builds byte-for-byte unchanged (OSCTAP_THROW(X) ≡ throw X).

Deferred follow-up:

• Non-throwing TryInit/validate entry point so a no-exceptions build can reject untrusted packets by returning an error instead of aborting. Until then, malformed input on an exceptions-off build is fatal — safe only on a trusted link (open networks should keep exceptions on and catch the Malformed*Exception types). See the security note in docs/EMBEDDED_PICO2W.md.
• A non-throwing realtime blob accessor (also tracked from the Phase 1 RTSan deferral).

C++ feature discipline and the realtime contract in ROADMAP.md apply.

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Non-throwing TryInit/validate landed in #20 (the deferred follow-up on this issue).

• ReceivedMessage::TryInit / ReceivedBundle::TryInit: the existing Init() validation, refactored to return a static error string (nullptr == ok) instead of throwing, setting the boundary members on success. The throwing Init()/constructors now delegate (Init = TryInit + OSCTAP_THROW on the returned error), plus ReceivedPacket::ValidateSizeNoThrow() — so there's one source of truth and the throwing/non-throwing paths can't drift.
• osctap::TryValidatePacket(data, size): recursive, non-throwing, non-allocating whole-packet gate (message, or bundle whose elements are all well-formed), bounded against deep bundle nesting. Returns nullptr iff the packet can be constructed and read in full without any OSCTAP_THROW firing — the gate to use before handling untrusted input on a -fno-exceptions build.

Tested: tests/OscValidateTest.cpp (hosted differential — the gate agrees with the throwing path across a valid message/bundle, every truncation, and several corruptions; clean under ASan/UBSan), and OscFreestandingTest extended to prove malformed input returns instead of aborting under -fno-exceptions. test4/test5 (throwing behaviour) unchanged.

With this, the freestanding/embedded-profile item is functionally complete. The only Phase-1-era item still genuinely open is OSS-Fuzz submission (#7).

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