docs: add docs-as-code process and one ADR per feature

docs/docs-as-code.md defines the workflow; docs/adr holds 31 ADRs
(retroactive, reconstructed from code and changelog) with a template and
index. docs/check-docs.sh validates ADR structure, index, cited code
paths, relative links and CLAUDE.md/AGENTS.md sync.

docs/adr/0004-centralized-network-profile.md

@@ -0,0 +1,25 @@
+# 0004. Centralize all chain constants in a network profile
+
+- **Status:** Accepted
+- **Area:** Chain
+- **Recorded:** 2026-11-03 (retroactive: reconstructed from code, comments and changelog)
+
+## Context
+
+Palladium is a Bitcoin-derived chain with its own address prefixes, BIP32 headers, bech32 HRP, genesis, ports and coin type, and it runs on mainnet, testnet and regtest. Scattered magic numbers make a wrong-network address or key a silent fund-safety bug.
+
+## Decision
+
+Define every chain constant once in `Core/Chain` (`ChainProfile`, `ChainProfiles`, `PalladiumNetworks`): address prefixes (P2PKH 55, P2SH 5), bech32 HRP `plm`, SLIP-132 headers, genesis hash, default ports (50001/50002), BIP44 coin type 746, maturity and confirmation policy, bootstrap servers and checkpoints. The profile is selected per network and passed into the code that needs it.
+
+## Consequences
+
+- A new network or parameter change is a one-file edit.
+- Testnet and regtest ship with empty bootstrap-server and checkpoint lists (TODO in code), so they rely on a user-supplied server and have no checkpoint anchoring (see 0006).
+- Mainnet currently reuses Bitcoin's genesis hash, as noted in the profile.
+
+## References
+
+- `src/Core/Chain/ChainProfile.cs`
+- `src/Core/Chain/ChainProfiles.cs`
+- `src/Core/Chain/PalladiumNetworks.cs`