Add INSTALL.md — end-to-end installation guide

Covers everything from git clone to accepting a first share, both
local dev (macOS/Linux) and production Ubuntu.

Sections:

  Overview — stack diagram, mode selection (solo / pps / pps-classic)
    with clear notes on which mode does what and when to pick which.

  Prerequisites — dep install commands for macOS (brew) and Ubuntu
    (apt + NodeSource + grpcurl).

  Part A — local dev: build + tests + regtest stack invocation.

  Part B — production Ubuntu: both the one-shot deploy-to-server.sh
    invocation AND a manual walkthrough (service user creation, git
    clone, make, npm install, DB init, systemd unit rendering with
    the @user@/@root@ substitutions).

  Part C — the drivechain stack: prebuilt binary URLs for
    bitcoind-patched, electrs, enforcer, Thunder. Config skeletons
    for each. Notes on the rest=1 requirement and the enforcer
    gRPC-first ordering constraint Thunder needs.

  Part D — proxy.conf per mode. Minimal working example for solo /
    pps / pps-classic, plus optional Redis broadcast block.

  Part E — dashboard install. Systemd drop-in pattern for env vars
    (PUBLIC_STRATUM_URL, ADMIN_USER, ADMIN_PASSWORD, THUNDER_RPC_URL,
    POOL_THUNDER_RESERVE_ADDRESS, POOL_PPS_SATS_PER_DIFF) with an
    openssl-generated password. Warning about admin-behind-plain-HTTP.

  Part F — payout worker install. Systemd drop-in + a dry-run
    invocation for validating the plumbing before enabling.

  Part G — first-run verification: 7-step checklist covering pool
    listening, journalctl for GBT connectivity, dashboard reachable,
    admin gated, first miner authorize, accrual landing in
    pps_credits, and the audit cross-check flipping ✓.

  Part H — install-time troubleshooting: hiredis missing header,
    GBT curl failure, enforcer rest=1 missing, Thunder gRPC race,
    pool_btc_address missing, address-in-use with the pkill -x note,
    invalid-thunder-address on authorize.

Every reference to external docs goes through the local guides
(OPERATOR_GUIDE, NONCE_AND_SHARES, CLASSIC_PAYOUTS, PPS_THUNDER,
VERIFY, scripts/regtest/README).

No hostnames / passwords / hostnames-of-specific-deployments; every
placeholder is < angle-bracketed > for the reader to fill in.

Author: rsantacroce