Phase 2 prep: freestanding/embedded profile groundwork + Pico 2W guide

[tap]

Lays the groundwork for Phase 2 ("Reach") with the one self-contained, keep-it-green item — the freestanding/embedded profile (no exceptions/RTTI) — motivated by a concrete demand signal: the Raspberry Pi Pico 2W (RP2350). The other Reach items (QEMU, Android NDK, multicast) stay demand-gated.

Closes the groundwork portion of #15; adds the guide for #16.

The build seam

New osctap/osc/OscConfig.h:

• Auto-detects OSCTAP_HAS_EXCEPTIONS from the compiler.
• Routes all 78 core throw sites through OSCTAP_THROW. With exceptions on it is literally throw X — hosted builds are byte-for-byte unchanged (the test4/test5 malformed-input asserts still pass). Under -fno-exceptions it calls a non-returning, user-overridable fatal handler (OSCTAP_FATAL_HANDLER, default std::abort()).
• OSCTAP_FREESTANDING drops the hosted-only facilities: <iostream>, the std::vector-backed OwnedMessage, and the std::string operator<<.

Also fixes a latent dependency: the outbound header used std::string relying on <iostream> to transitively include <string> — now explicit.

Profile + CI

• OSCTAP_FREESTANDING CMake option builds tests/OscFreestandingTest.cpp with -fno-exceptions -fno-rtti (flags PRIVATE to that target, so the exception-based tests are unaffected). The test serializes then parses a message through the checked accessors (proving OSCTAP_THROW compiles under -fno-exceptions) and the realtime *Unchecked path.
• New matrixed freestanding CI job (GCC + Clang).

Docs

• docs/EMBEDDED_PICO2W.md — Pico SDK + lwIP integration recipe (use osc/ core, replace ip/ sockets with lwIP UDP), receive/send snippets, no-heap RT checklist, and the untrusted-input/DoS trade-off: with exceptions off a malformed packet hits the fatal handler, so open networks should keep exceptions on and catch — the non-throwing TryInit/validate to fix this is tracked as deferred work in Phase 2: Freestanding / embedded profile (no exceptions/RTTI option) #15.
• ROADMAP.md / docs/STATUS.md — mark the groundwork landed, restate the remaining Reach items as demand-gated, fix the stale status line, add landmines for the new seam.

Verification

• Hosted suite green: C++17 + C++20, GCC/Clang/MSVC matrix, ASan/UBSan (4/4).
• OscFreestandingTest builds + runs under -fno-exceptions -fno-rtti on GCC and Clang, C++17 and C++20.

Follow-up (tracked)

Phase 2 issues opened: #15 (freestanding), #16 (Pico 2W), #17 (QEMU CI), #18 (Android NDK), #19 (multicast). The "Phase 2 — Reach" milestone object is an owner action — the GitHub MCP server exposes no milestone-creation endpoint.

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