feat(net): AcDream.Core.Net scaffold + ISAAC keystream (Phase 4.1)

First step of Phase 4 (networking). Adds a new AcDream.Core.Net project
for the AC UDP protocol implementation and a matching AcDream.Core.Net.Tests
project. Keeps networking isolated from rendering and the dat layer,
which also keeps the AGPL-reference-material hygiene cleaner.

AcDream.Core.Net/NOTICE.md documents the attribution policy: we read
ACE's AGPL network code (and holtburger's Rust ac-protocol crate) to
understand AC's wire format, but we reimplement everything in acdream's
own style. Wire-format facts aren't copyrightable; specific code is.

This commit adds one component: IsaacRandom — AC's variant of Bob
Jenkins' ISAAC PRNG, used to XOR a keystream into the CRC field of
every outbound packet for authentication. Clean-room reimplementation
based on reading:
  - references/ACE/Source/ACE.Common/Cryptography/ISAAC.cs (AGPL oracle)
  - Bob Jenkins' public ISAAC algorithm description

Implementation notes:
  - 256 uint32 mm[] state, 256 uint32 rsl[] output buffer, a/b/c regs
  - Initialize() runs 4 golden-ratio Mix() warmup rounds then two fold-in
    passes over rsl[] and mm[] (fresh instance → both start as zeroes)
  - AC variant: seed is exactly 4 bytes, interpreted as little-endian
    uint32 assigned to a = b = c before the first Scramble()
  - Scramble() produces 256 output words in one pass; Next() consumes
    them backwards from offset 255 → 0, re-scrambling at offset -1
  - Test seed 0x12345678 matches ACE's reference output byte-for-byte
    across the first 16 values (golden vectors transcribed from a
    throwaway oracle harness that compiled ACE's ISAAC.cs and printed
    its output; the harness was deleted after extracting the values)

Tests (5, all passing):
  - Next_Seed12345678_MatchesAceGoldenVectors: 16 golden uint32 values
  - Next_TwoInstancesSameSeed_ProduceIdenticalSequence: 1000 outputs
  - Next_DifferentSeeds_ProduceDifferentFirstOutput
  - Next_512Calls_SpansTwoScrambleBatches: >400 distinct values in 512
    outputs (catches all-zero / stuck-at-one bugs at scramble boundary)
  - Ctor_ShortSeed_Throws

Both test projects still green: 77 core + 5 net = 82/82.

Phase 4.2 (packet framing + checksum) next.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/AcDream.Core.Net/NOTICE.md

@@ -0,0 +1,25 @@
+# AcDream.Core.Net — Protocol Reference Attribution
+
+This project implements the Asheron's Call network protocol for acdream.
+The wire format is a fact about the retail AC client/server and cannot be
+copyrighted, but the specific implementation details and structure conventions
+are informed by reading the following open-source projects:
+
+- **ACE** (ACEmulator / `references/ACE/Source/ACE.Server/Network/` and
+  `references/ACE/Source/ACE.Common/Cryptography/ISAAC.cs`) — AGPL-3.0.
+  The authoritative server-side reference for packet framing, ISAAC keystream
+  generation, fragment reassembly, and GameMessage opcodes.
+
+- **ac-protocol** (holtburger's Rust crate) — AGPL-3.0. Confirms the handshake
+  works in practice.
+
+No code is copied from these sources. Algorithms are reimplemented from scratch
+in acdream's own style after reading and understanding the reference. Where
+golden test vectors are derived from running the reference implementations,
+those vectors are factual outputs of the algorithm, not copyrighted expression,
+and appear only in test code.
+
+If you plan to redistribute acdream outside personal use, consult a lawyer
+about the interaction between our chosen license and the AGPL heritage of the
+references we read. (We didn't at the time of writing — see the project-level
+licensing notes.)