feat(core+host): negotiate ChaCha20-Poly1305 as the session cipher for soft-AES clients

Lifts the ~100 Mbps decrypt ceiling on clients without hardware AES — the
armv7 soft-AES targets (webOS TVs), where AES-128-GCM resolves to fixsliced
software AES + software GHASH (~50-100 cpb) while ChaCha20-Poly1305's ARX
construction runs ~10-17 cpb portable, a 4-7x lift that PyroWave-on-TV needs
(design/chacha20-session-cipher.md).

Phase 1 (core crypto, no wire change): SessionKey merges cipher choice and
key material (invalid combinations unrepresentable, zeroize + redacted-Debug
discipline kept); SessionCrypto dispatches both aead-0.5 ciphers per call —
the salt||seq nonce scheme, per-direction salts, seq-as-AAD and replay
window carry over verbatim (same 96-bit nonce / 16-byte tag, const-asserted).
Config.key becomes SessionKey; validate's zero-key rejection follows the
active variant. The C ABI keeps its fixed 16-byte key mapped to AES — no
ABI_VERSION bump.

Phase 2 (negotiation): VIDEO_CAP_CHACHA20 (0x40) — support-plus-request in
one bit, the VIDEO_CAP_444 precedent. Welcome grows cipher@68 +
key_chacha@69..101, emitted only when non-zero so an AES session's Welcome
stays byte-identical to the pre-cipher form; decode is fail-closed (short
key or unknown id -> Err, never a silent AES fallback). No WIRE_VERSION
bump; downgrade resistance inherited from the pinned-TLS control channel.

Phase 3 (host): grant only when the client advertised the bit and the
PUNKTFUNK_CHACHA20 kill-switch (default on, documented) allows; fresh
32-byte per-session key from the same RNG discipline, legacy key field
stays independently random; resolved cipher logged at session start.

Verification: seal/open suites parameterized over both ciphers + a
cross-cipher tamper case; Welcome roundtrip/truncation/fail-closed tests;
ChaCha lossy-loopback soak (loss/replay is cipher-independent); bench
gains _chacha20 series (AES ids unchanged for CI history) — host-side
sealing line-rate-trivial on both x86 (~640 MiB/s) and Apple Silicon
(~535 MiB/s). punktfunk-probe drives the interop matrix via
PUNKTFUNK_CLIENT_CHACHA20=1 and logs the negotiated cipher.

Phase 4 (pf-webos pin bump + unconditional cap bit) follows the next
core release.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs-site/content/docs/configuration.md

@@ -90,6 +90,7 @@ See your desktop page ([KDE](/docs/kde), [GNOME](/docs/gnome)) for when to set t
 | `PUNKTFUNK_FEC_PCT` | `N` (percent) | Forward-error-correction redundancy for lossy links (the default is sensible for a normal LAN). Higher = more loss-resilient, more bandwidth. |
 | `PUNKTFUNK_10BIT` | `1` · `0` *(default on)* | HEVC Main10 / HDR. **On by default** — the host permits 10-bit; a session goes 10-bit only when the client advertises it (behind the client's HDR setting). Set `0` to force 8-bit. Windows host, plus the Linux **GNOME 50+ GameStream desktop mirror** (`PUNKTFUNK_VIDEO_SOURCE=portal`, mirrored monitor in HDR mode — check with `punktfunk-host hdr-probe`). Linux **virtual displays** (native protocol, GameStream default) stay 8-bit: Mutter's virtual-monitor screencast is SDR-only upstream. |
 | `PUNKTFUNK_444` | `1` · `0` *(default on)* | Full-chroma HEVC 4:4:4 (Range Extensions) — sharper text/desktop, no chroma loss. **On by default** on the host; the client's own 4:4:4 setting (default off) is the real switch. Set `0` to force 4:2:0. **punktfunk/1 native only** (Moonlight stays 4:2:0), HEVC-only, honored only when the client advertises 4:4:4 **and** the GPU supports it (probed; NVENC is the validated path — VAAPI/AMF/QSV decline). Independent of 10-bit. |
+| `PUNKTFUNK_CHACHA20` | `1` · `0` *(default on)* | ChaCha20-Poly1305 session encryption for clients without hardware AES (old ARM TVs, e.g. webOS), lifting their ~100 Mbps software-AES decrypt ceiling. **On by default** on the host; a session uses it only when the client requests it — everyone else stays on AES-GCM. Purely a performance choice (both ciphers are full-strength); set `0` to force AES-GCM for all sessions. |
 | `PUNKTFUNK_PYROWAVE_MAX_MBPS` | `N` (Mbps) | Cap the [PyroWave](/docs/pyrowave) Automatic bitrate pin, for a host on a link that the open-loop pin can outrun (e.g. 4:4:4 + HDR at 5120×1440@240 pins ~5.3 Gbps, over a 5GbE link). Unset = no cap. Only affects Automatic (bitrate `0`) PyroWave sessions; an explicit client bitrate bypasses it. |
 | `PUNKTFUNK_DSCP` | `1` | Opt-in DSCP / `SO_PRIORITY` QoS tagging on the media sockets. No-op on the wire on Windows without a qWAVE policy. |
 | `PUNKTFUNK_OH264_THREADS` / `PUNKTFUNK_OH264_GOP` | `N` | Software (openh264) encoder tuning: encode threads (default 2 — latency over throughput) and GOP length (default 0 = encoder-auto). Only relevant with `PUNKTFUNK_ENCODER=software`. |