refactor(theseus/aegis): rename bundle bchwallet→aegis + retire standalone siawallet

Cleans up the naming that leaked from the wallet's origin story (BCH-only)
into the actual bundle layout. Aegis is one integrated addon now:
- Bundle folder: TheseusNavigator/bundled-addons/aegis/ (was bchwallet/).
- Addon id:      "aegis" (was "bchwallet"). Vault-derive still accepts
                 legacy "bchwallet/*" and "siawallet/*" paths via the
                 absorbs list, so no on-chain funds move.
- Version:       0.4.0 (bumped to trigger seedBundledAddons's reseed).
- Retired:       TheseusNavigator/bundled-addons/siawallet/. Sia is
                 folded into Aegis as a chain adapter (lib/sia/*.js
                 already in-tree) and Aegis's manifest lists siawallet
                 under absorbs so pre-Aegis SC keys derive identically.

main.js migrateAegisRename() runs before seedBundledAddons on every
launch. First run does the move; subsequent runs are no-ops:
- addons/bchwallet/  -> addons-backups/bchwallet-migrated-<stamp>/
- addons-data/bchwallet.json COPIED to addons-data/aegis.json (kept
  copied not moved so a downgrade to 0.3.x can still boot).
- addons/siawallet/  -> addons-backups/siawallet-migrated-<stamp>/
  (addons-data/siawallet.json left untouched — its walletdUrl is
  per-user config Aegis's Sia wallet takes fresh via Settings).

settings.html Aegis update card now matches either "aegis" (new id) or
"bchwallet" (pre-rename) so upgraders coming from 0.3.x see the same
one card while the OTA endpoint's next signed bundle catches up.

Internal purpose paths inside index.js/chain-*.js are unchanged —
LEGACY_BCH_PURPOSE stays "bchwallet/mainnet/0" and every purposePrefix
still starts with "bchwallet/*". The addon absorbs its own former id,
so those paths keep resolving to the same seed the shipping Aegis has
been using since 0.3.14.

addon.json

@@ -1,13 +1,13 @@
 {
-  "id": "bchwallet",
+  "id": "aegis",
   "name": "Aegis Wallet",
-  "version": "0.3.1",
+  "version": "0.4.0",
   "description": "Multi-chain wallet (BCH, BTC, TRX, ETH, SOL, SC, DGB) derived from your Theseus vault. Dapps get window.bitcoincash on .x sites; window.tronWeb / window.tronLink / window.ethereum / window.solana on any https page.",
   "author": "Silent Mode",
   "icon": "🛡",
   "main": "index.js",
   "capabilities": ["sidebar-panel", "vault-derive", "page-inject", "approval-modal"],
-  "absorbs": ["siawallet"],
+  "absorbs": ["bchwallet", "siawallet"],
   "page-inject": {
     "preload": "wallet-inject.js",
     "origins": ["https://*/*"]